It most likely has to do with Ukraine funding. Mcarthy pushed forward a temporary spending bill that excluded it. The neocons were furious, and the neocons effectively control both parties. Watch what happens. I guarantee you that 24 billion will get passed regardless of who is speaker now, and the message was if you fuck with our Ukraine funding you will be removed.

This isnt defending Mccarthy. He was hardly making a principled stand and would have ensured it passed eventually, but the neocons are concerned that it would set a precedent, embolden war critics and make funding their Ukraine blood bath further even harder in the future.