Never seen this before, myself. Can't see how firing alone would do this. If they are repro grips and a little out of spec, perhaps they were forced on to fit and thus already (unobservably) damaged on that side?
That way, as the cracked sections may have already been there when the magazine was inserted and possibly thus jammed up. The only way I could see how they could be ejected as you state, while firing. Can't really imagine any other way off hand how this would happen.
At least as repros they don't cost too much to get others and replace them. I would do that for shooters anyway as I would not want to wear out originals. Good safety reminder to have safe shooting glasses!
Judging from where the grips broke, I would say it was a user problem, not a gun problem.
The vertical interior rib near the front of each grip panel is a shallow dovetail, and --when properly assembled to frame--engages a mating dovetail on the frame. The grips have to be correctly engaged at the front first, before the rear of each panel is pivoted down to meet the other at the backstrap. When properly installed, the grips will rest flat on the frame, with no gap at the front edge.
Failure to do this leaves the panels unsupported by the frame and recoil will break them.