What seems to have happened is the machine was making a full depth pass, didn’t quite make the turn at the bottom of the circle, and because the stock was thicker than the flutes are long, was guided by the stock until the endmill broke.
Avoid slotting (cutting a toolpath as narrow as the endmill w/ a single set of stacked/successive paths) and add geometry so as to cut as a pocket down to at least tab depth or the penultimate pass:
and/or
and consider leaving a roughing clearance and taking a finishing pass.