I guess the best substantive answer I could give you is this:
The only way to have your question answered is to talk to the crew. I'm not sure that right after the flight is the best way to do it, but I can't see any other way for you to talk to them. I think the best thing to do is just put it up to the idea that we may never know. Like you said, 90% of the landings were on the other runway, so there may be a good reason this was one of the 10%. Another thing, runway closures are common as you may know and it could have been getting checked for FOD. Also, the approach may have been notamed or temporarily closed. There are a million good reasons, probably the best one was that they were PIC and SIC so they were the only ones with the whole story T/O to landing so they have the final say. I would just write it off as a bad landing and maybe, "MAYBE" a bad chain of errors that got them on the ground just in time.