That sounds like the steppers missed some step. The noise you heard is kinda normal when this happens.
Ofcourse there sure could be more that may be going on. Pay closer attention and try to get more info if it happens again.
The HSS bits will give/flex more than a carbid bit before breaking. ANd it possibly being a little dull would add to the forces. I dont know a lot about feed/speeds to help with possible setting problems for the metal being cut, but it may just be as simple as changing the settings and all be good.
The fact that you did the same job without problems could be telling you there is a problem, or it could have just been the added forces of the dull bit. If you where on the edge with your speeds & feed this might of just pushed it over the limits.
I would check all connections, and move the Y axis by hand after removing the stepper so that you can check for binding of the axis. This could be a gib getting to tight or maybe even debree or the lack of oil. Or even something like a bent screw, or bearings getting tight. But if you remove the stepper you should be able to tell if there is any rough spots in its travel. It also could of been noise in the system. If you have moved any wiring, added anything electrical to or around the machine lately then give this some attention. ANy removable connections could be getting corrosion and causing problems, clean if needed.
There are many things that could be at fault, but going over the basics would be a good place to start checking.
Jess