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Your code constantly updates because nPts is local to a module enclosing the Manipulate. If the Manipulate enclosed the module, your code wouldn't work as you would like, but it wouldn't be constantly updating.

Here is a much simpler example showing the same behavior.

Module[{n = 1}, Manipulate[Row[{"n = ", n++}], {t, 0, 5}]]

constantly updates, but

Manipulate[Module[{n = 1}, Row[{"n = ", n++}]], {t, 0, 5}]

doesn't.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, 
     update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True]]

This is much simpler than your code, but still performs the simulation you trying to implement. Give it a try and see if it satisfies your needs.

Your code constantly updates because nPts is local to a module enclosing the Manipulate. If the Manipulate enclosed the module, your code wouldn't work as you would like, but it wouldn't be constantly updating.

Here is a much simpler example showing the same behavior.

Module[{n = 1}, Manipulate[Row[{"n = ", n++}], {t, 0, 5}]]

constantly updates, but

Manipulate[Module[{n = 1}, Row[{"n = ", n++}]], {t, 0, 5}]

doesn't.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, 
     update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True]]

This is much simpler than your code, but still performs the simulation you are trying to implement. Give it a try and see if it satisfies your needs.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, 
     update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True]]

Give it a try and see if it satisfies your needs.

Your code constantly updates because nPts is local to a module enclosing the Manipulate. If the Manipulate enclosed the module, your code wouldn't work as you would like, but it wouldn't be constantly updating.

Here is a much simpler example showing the same behavior.

Module[{n = 1}, Manipulate[Row[{"n = ", n++}], {t, 0, 5}]]

constantly updates, but

Manipulate[Module[{n = 1}, Row[{"n = ", n++}]], {t, 0, 5}]

doesn't.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, 
     update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True]]

This is much simpler than your code, but still performs the simulation you trying to implement. Give it a try and see if it satisfies your needs.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, update = False; pts = updater[]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True],
  Initialization :> (
    vels[n_] := RandomReal[{-1, 1}, {n, 2}];
    updater[] := (pts = MapThread[(#1 + #2) &, {pts, vels[Length@pts]}])
  )]

Give it a try and see if it satisfies your needs.

I would take a different approach. I wouldn't use locators to add more particles to the simulation; rather I would use a ClickPane. Nor would I use a slider to generate update events; rather I would use a Button

Manipulate[
  If[update, 
     update = False; pts = Plus @@@ Transpose[{pts, RandomReal[{-1, 1}, {Length@pts, 2}]}]];
  ClickPane[
    Framed@Graphics[{Blue, PointSize[0.02], Point[pts]}, PlotRange -> {{-5, 5}, {-5, 5}}], 
    AppendTo[pts, #] &],
  {{update, False}, ControlType -> None},
  {{pts, {{0, 0}}}, ControlType -> None},
  Button["Update", update = True]]

Give it a try and see if it satisfies your needs.