I have seen similar things happen when someone I am skiing with gets on a different pair of skis. We took our small Boy Scout troop skiing quite often, and parents were always welcome. A lot of times I would provide skis (or someone else would). We were dealing with people who haven't skied in a while, typically. the wider the borrowed ski, the more trouble people had with it. What I chocked it up to was "pivot style" turns versus carved/slaved turns using your edges.Old school straight skis needed rotary pivot to get them to come around at some point in the turn. Same thing happens when an East Coaster goes out west for the first time, gets in deep snow and can't pivot/slide their turns. My youngest brother still prefers straight skis and pivots his skis, then edges them to finish the turn. That is "his style". When he gets on mordern wide skis, he usually doesn't like em, under 80mm and he can work his style even with shaped skis.
I am not sure how old the B4's are, but the Kore's didn't fit his skiing style for sure. Last time I skied my old straight skis, I almost fell over on the first turn. I forgot to pivot them and they didn't come around like I was expecting just from rolling them on their edges. My upper body was way out of sync with my lower body.