Leo had pushed them extra hard during their morning practice. It was Mikey’s fault. He started goofing off and when horseplay is added to weapons training accidents happen. The resulting welt on Donny’s arm was nothing compared to the black eye Mikey had, thanks to Raph.
During the time that Leo and Mikey were gone was like a honeymoon for Raph and Don as well. But, when they returned, things went back to normal. Training resumed and Mikey went back to making trouble, mostly for Raph and Don.