I've not come across mansplaining much irl but when I have I'm pretty sure it's been for sexist reasons. However, I read this article recently and it made me think.
I think some mansplaining is definitely sexist but could some cases be communication failure due to men sometimes using different ways to bond with each other than women do with their same sex friends? Other cases could be men using it to show dominance, but not necessarily precisely in a sexist way as they may do this with other men too.
YABU - no this generally isn't the case.
YANBU - yes it is, at least sometimes.