I don’t have enough knowledge about GFCI. But I did some Google search and my understanding is that it trips when the difference between the supplied current and returned current is more than, say 6mA. This is often due to leakage current, unexpected shorting to ground, or anything that causes the current to leak out without returning. I can’t think of an obvious reason for your case, but I suspect if there is a wire that is worn out or exposed, it can result in leakage which trips the GFCI.