It was a dire situation, either drop Donnie off an extremely tall building leading to his death or allowing 'the Shredder' to takeover the world with dangerous mutation by bugs. Normally this would be an easy choice but sadly Donnie didn't have his battle shell he could use to fly with so he couldn't protect himself and Raphael's powers weren't working so he couldn't save him, before any of the brothers could react Leo chose the bugs

"So be it.." Shredder growled, dropping Donnie off the building. "DONNIE!" Raph and Mikey yelled, All the brother's eyes widened in horror, did Leo just choose to save the city over his own brother..? Tears began filling Donnie's eyes as he began falling down the extreme tall building, clenching his chest in heartbreak. It felt as if the world was ending right then and there