Why did Bakugo do that? Was it because he spent with Deku and began to see him as a friend?
For some reason, Horikoshi doesn't fully reveal this part.
He did explain it perfectly. In Chapter 284 Bakugou reveals his true feelings and motives. He tells All Might why he bullied Deku and that he actually trusts him, remember? And before taking the hit for Deku he thought: "I don't know why I did this, my body just moved by itself". It shows that he unconsciously felt very attached to him and sees him as a significant friend (He never showed or knew it, because he wanted to hide his own insecurity). Plus he also knows Deku is the only one who can defeat Shigaraki. So if Deku dead, everyone dead.
Btw. There is also an interesting theory about Bakugou being the second user of one for all. That might also be a reason why his body reacted like that, if the theory is true.










Kekekeke~
Thts me lol...this activated my inner fangirl