I think that’s a coincidence. The real deadline is June 1, because there is another deadline in WV for independant candidates to file for the ballot on Aug 1, and those applications can be challenged if the candidate changes parties in the two months prior to the application. So by doing this as late as possible in May, he preserves the right to run for Governor or Senate as an Independant.
And like it or not, if he ran for either of those seats, he would win, and would be better than any Republican who might have won in his place. And if he ran for Senate and won, there is a very good chance that the party he decides to Caucus with will determine which side controls the Senate. So if he wins, he wil still be very popular with the establishment on both sides.
You can expect Chuck to give him any committee chairmanship he wants if it means Chuck can get to 50-50 (where committees are equally balanced, and the VP breaks ties) or 51-49 (which gives Democrats the majority on committees regardless of who is the VP). If Democrats keep all their seats then also gain TX or FL, then Manchin’s vote is not as critical and Chuck is in better position to make him irrelevant.