So, I was flying normally, CYYZ to KBOS (I know short but I love it so much) and when I reached top of decent, the auto throttles went to full power and over speeded the aircraft. I disengaged them, slowed, reengaged them and the problem didnt return. This a glitch or something that is supposed to happen?

Look at the FMA. If the auto-throttle setting shows hold and you have your thrust levers at maximum (I assume you're using hardware for this), exactly that happens. When it's in hold mode, it'll hold your current thrust setting. You'll want to manually bring your thrust levers back to idle.

8 hours ago, NickFlightX said:

So, I was flying normally, CYYZ to KBOS (I know short but I love it so much) and when I reached top of decent, the auto throttles went to full power and over speeded the aircraft. I disengaged them, slowed, reengaged them and the problem didnt return. This a glitch or something that is supposed to happen?

As Kevin mentioned, this is due to (correct) AT behavior. The difference is that the real AT drives the throttle levers back to IDLE through servos in the real plane. Your hardware likely doesn't have servos to do that, so you will have to manually do it prior to reaching TD. Otherwise, well, the sim is logically going to put the throttle where you're telling it to: full.