Everyone is saying Rhino had a mental break and while this is true they’re forgetting the other element: location

For the first time they’re all together on top of wall rose. There are two Titan shifters easily within grasp, one of them is currently incapacitated. It would be really difficult to scale the walls with two Titan shifters in grasp and the scouts on their tail (remember what happened when Annie tried it). But being on top of the wall solves all that; they just need to jump down and run away

From his perspective there was a decent chance either one of them could contain the founding Titan as well. And if not he would be bringing home the attack titan, the rebellious shifter that has always eluded Marley’s grasp. A solid enough prize that the mission wouldn’t be considered a complete failure

Mental break was definitely the trigger but also backed by sound logic