It's more like they're both similar to Frutiger. And Myriad is barely 2 years older than Segoe, and it was developed by Agfa not MS, so it's not likely that it was copied. While Arial was kind of a crappy font, MS's new stuff (Consolas, Calibri etc.) has been pretty nice.
They're all Humanist fonts so there's a fair amount of family resemblance. So take the school of thought that finds Helvetica boring and corporate and apply to the new Segoe-based Microsoft logo. One thing I find interesting is that there was some legal brouhaha over Segoe a few years ago which appears to be all resolved but still makes it a little surprising to me that they'd go so all-in in such a visible context.
