It looks as though Tumblr might have more than just one suitor. According to a Forbes report, Microsoft and Facebook are also interested in the blogging network. However, these talks would be irrelevant if Yahoo makes an offer that is accepted by Tumblr. If Yahoo's scheduled event is to announce a partnership, these talks clearly haven't worked.
Of course, the event could be used to announce an official offer for Tumblr without actually having an acceptance. However, Forbe's sources suggest that the Yahoo/Tumblr talks are progressing rapidly and well. Marissa Mayer would still need to secure an offer from Yahoo's Board of Directors before an official offer could be made.
Yahoo is probably attempting to get a lockup agreement that would prevent Microsoft, Facebook, and Yahoo from entering a bidding war.