This same thing happened this year for that exact week. We were there May 5 - 12 and had a REALLY hard time booking. We did it sort-of last minute (about 75 days out or so)...the only rooms available at any
DVC resort were 1 BRs at SSR. We were not DVC members at that time (but had rented before), so we thought we would see what else was available at the other onsite resorts. The only rooms anywhere were at ASMu, I believe (might be off here, but I know we couldn't get into any of the Mods...I checked on some of the deluxes and could not get in, but I did not check all of them). We ended up renting the 1 BR at SSR...which was FAB and helped us make our decision to buy there.
Needless to say, I was really stressed thinking that the prarks would be ultra crowded like they are during Christmas week. In fact, if you search these boards, you will find several threads that start just like this one..."What is going on in early May??" There was much speculation, but no real concrete answers...same thing you are getting here....Flower and Garden, Mother's Day, etc. My own two cents to add to those things are that many colleges get out May 1 - 5, so that may have an impact.
In any case, while the parks were not empty by any means, they were certainly manageable. We didn't wait long for anything (we planned our park time well, though...had a strategy for each park before arriving), but all of the TS restaurants were PACKED. We made our ADRs, so we were all set, but we still had to wait an average of 15 minutes for each of our table times...plus we wanted to make one change, and we could not get in for dinner anywhere last-minute.
So, I don't really have an answer for you....except to say that what you are noticing for that week in May has happened before.
Happy planning!
P.S. It was definitely very hot that week....not "warm" -- out and out HOT.