Hi Del
I can't remember what the weather was like when you were at work, but I think it is fair to assume you are an approach controller at TC, so some possible answers are:
1. ANTI ICE. If it is switched on (engine or wing) half way down the final slope, this can give an increase in speed of a few knots from a previously stable situation.
2. TAIL WIND COMPONENT. If the wind is mainly across the approach with a slight headwind component which becomes a slight tailwind lower down, then an airspeed increase is possible as the flight seeks to maintain the 3 degree path.
3. TURBULENCE/GUSTY WIND. I am not sure what the update rate of your mode S is, but if the ASI needle is moving around quite a bit due to turbulence then I think that this may give the impression of an increase in airspeed if the maximum value happens to be the one that the transponder transmits.
4. INVERSION LAYER. Looking at the time of your posting it looks as if you may have been working in the late evening (afternoon with an eg or D2 is it!?). If there was no cloud, then an inversion layer may have developed which decouples the wind near the surface from that higher up (2000' say). Thus at different levels there may be different tail/headwind components and as the aircraft is bound by Newtonian physics, the airspeed may change as the layer is transitioned.
Not a definitive list. I am sure someone will be able to think up some more reasons. Hope it helps.
G W-H