Long story short, Leo was messing around in Donnie's lab, like a dum-dum. In all honesty, it's Donnie's fault for leaving the door open. He ended up turning into a baby turtle again.

While Donnie and the others try to come up with a cure for Leo, he's content playing around with a ball in the make shift play pin. Of course you have to watch him. We can't leave him hear alone. He gets in trouble as a teenager imagine how badly he'd mess up in his current state, the little devil.