I have a Bushmaster Superlight with about 1500 rounds through it.  OCCASIONALLY, when the bolt is back and locked and I shove in a new magazine, the bolt will slam home chambering a round.  Other times, I have to depress the release to make it go home.  What is happening here?

My gues is that i's simply the shock of the mag getting slamed in there that's giggling the hold opeen device/bolt enough for it to disengage...
'Same kinda thing that happens when you tap the butt of your rifle on the ground... often it's enough to release the bolt catch...

The bolt catch obviously is activated by the follower of your magazine, hence the last round hold back. Have you tried different magazines with different followers to see if the same thing happens?
Also, have you cleaned and oiled your bolt release and magazines to make sure they're not sticking? The bolt catch might also be out of spec, so if you may want to try and replace it. What Girlieman says is certainly possible, but I've never had it happen on my rifle, and I can understand how it can be annoying.

The bolt catch could be getting 'hung up' and not pivoting fully to allow a good bite of the bolt face... Try cleaning the bolt catch pivot areas with action cleaner then oil those areas real good... After oiling, by hand, manipulate the bolt catch several times to work the oil into the pivot pin (roll pin) area...