That’s interesting, Geordie, because I just came across that story this week as I was Pesach cleaning (it was on some pages that hadn’t gotten ripped out of R’ Yaakov zatsal’s biography), and I’m pretty sure it said that he didn’t know the reason for the minhag. If I come across it again, I’ll double check.
I found it interesting that on the one hand it was so important to him to keep minhagim that he kept it even without knowing the reason and even though it came from his grandmother (as opposed to father or grandfather) and on the other hand, he gave it up (at least temporarily) so spare his wife’s feelings.