As it stands, here are some possibilities... they want to keep Yabusele with the team instead of stashing him, they might have a good outlook in keeping Zeller (maybe in a 1yr deal), maybe a trade is coming and they needed the cap space.
From a C's standpoint, those might make the most sense to me. Of course, maybe they did indeed feel pressured to sign Al Horford right away. But, even though I feel this was a premature thing to to, I'll have to wait and see what else happens before we can judged what occurred here. Ainge and his CBA Guru team are very good at their job, and they always seem to milk the most they can from their cap room situation.
So, will be interesting to see in the end what motivated this decision and so soon into FA.