으니까 그러다

that's why he's doing that

"-(으)니까" gives a reason and 그러다 stands in for the behaviour being explained, so the whole thing means "that's why". It is the answer to an unspoken "why is he like that?" — he's like that because he's singing at the wedding, you're saying that because you haven't seen "Your Name", he's only saying it because he isn't interested. It very often ends in -지 or -잖아, the endings for pointing at something the listener should already have worked out.

Register: casual spoken