Keisuke was your bully from the start, in fact. It's not that you did something wrong, but Baji just felt like bullying you. No one knows why, but that is one of Bajis traits - no one knows what is he thinking.

Baji also started acting a bit weirdly recently. He bullied you still, but was a bit more gentle

So one of the following days, when you were just chilling in English class, teacher said that from now on you will sit with Baji. You sighted in despair, but Baji did grin.