1. Ashmit, Shweta and Sameer by clarifying things with each other and publicly have exposed MT as a manipulator and gossip to the entire household. If he had stayed then he would've had no friends in the house.
2. The divide between both the groups is becoming thinner, so he has lot less scope for manipulating people. By not waiting for Salman to declare the evictee, he's made sure that he has some amount of public sympathy.
3. He had already decided that he would nominate Sameer, he was looking for an excuse which he found when Sameer took his name as a likely candidate today.
Rest was all simple planning on his part which he had already decided beforehand, what we saw was some very fine spontaneous acting.