Uberto Alberti. Trusted friend of the Auditore family. According to the history books, this guy was a saint. He prosecuted murderers, rapists, the worst criminals. One of the best lawyers in Italy, he won every case, in spite of the fact he was self-taught.
Now, I found a backdoor into the Templars' database server, I've been combing it for some kind of smoking gun. I found it. Apparently, Uberto's family was evicted by the Medici bank. Uberto's been aching for revenge ever since, and the Templars promised him support.
Ezio's father was standing in the way AND Uberto was jealous of his influence over the Florentine government. Two birds with one stone, isn't it? It looks like Uberto used his election to the Signoria as his opportunity to strike.