Bathroom lights went out, but breaker isn't tripped
The first part of the problem is that yes, your lights are downstream of a GFCI. Which is not usually required. It may be that someone was in the mode of "I better put GFCI wherever I can and protect everything in the bathrooms, including the light", or it could be that there is another receptacle (bathroom, garage, outside, kitchen) on the same circuit that actually needs GFCI protection.

The second part is right there in your question:


  As I was pushing the cover onto the can, all the bathroom lights went out.


You mentioned a ground wire, so this sounds like more than just a simple screw in bulb replacement. But whether that new ground wire was involved or not, it sounds like you pinched or shorted a wire when putting the cover on the can. That could easily cause a ground fault and trip the GFCI. Then you took things out, double-checked everything, etc. and when you put it all back together, everything was OK. Except that you had to find the GFCI and reset. Once you did that, all the lights worked.