I dunno, maybe they just want to make sure the intersection is clear before proceeding, since they are a high risk area for accidents. Or Maybe they slow because they are looking for a certain street?!
I myself hate the inconsistency of the lights at intersections! Some amber lights in my city are so short, only a couple of seconds, literally, so if you are approaching and intending to go through it, it will turn red almost before you are past the crosswalk. That amounts to a big fat fine if you're caught (by a cop car or on camera). Then other ambers are very looong. I have noticed up in these parts, many, many people do not stop for ambers, so knowing how aggressive and impatient other drivers are, maybe that's part of why some people will slow on approaching an intersection. And also, at many intersections now, there's a countdown at most pedestrian crosswalks. I love them because as I am approaching, I can clearly see how many seconds before the amber light, so I can plan better. Then there are some intersections in which I have the green light, but the crosswalk has the red hand and it stays red for quite a while before the amber light. So, it's impossible to know exactly when it'll change, like at those "count-down" ones. I do try to slow down a wee bit, just as a precaution. I've seen enough drivers run amber and solid red lights, turn left when oncoming traffic is too close (I've had to hit the brakes a few times for those morons). Then, out in the country, there's no crosswalk indicators, at all. So you're approaching at a high speed, 55 or 60mph, and there is NO warning when that green light up ahead will change. I hate that! I always slow, just in case, especially if there's anything obstructing my view of all traffic going every which way.
So, those are my thoughts as to why.