Ever since Leo had became leader, things changed. When it was painfully obvious Leo wasn't taking his responsibility as serious as it should be, Raph instantly got bothered by this and tried confronting him, but it only ended up with the two fighting, longer than there normal bickering.
Later, they found themselves fighting more often, and it was harder for both Donnie and Mikey to deal with.
(PM or comment "May I?" if you wanna rp or whatever)