When McCarthy chased after the Speaker position, he had to cut a deal with the real, real, real wacky fringe of his party (House Freedom Caucus) to secure their votes for him after 12+ rounds of voting. One of the biggest things he gifted them so he could hold the gavel was the ability for a single member to be able to make a motion to vacate him from his position.
If he does anything the HFC disagrees with, it is likely they'll move to oust him; that includes any kind of Continuing Resolution that would require democrat's support to pass. I'm not saying it's guaranteed, but a shutdown at the end of this month has the highest probability we've seen since the last shutdown.
Whatever HFC wants to make the shutdown about will likely depend upon whatever complaints the HFC members want to pull from their asses; unknown at this time if they'll have a unified front on their messaging, but the lead up so far has just been them bitching about all non-military discretionary spending needing to be cut across the board. They are also the ones that have said government employees are paid too much and should have our benefits cut. So maybe the wall, maybe Biden impeachment, maybe cutting all spending to beyond austerity measures, maybe something else, maybe all of the above. That's the thing: the only prediction you can make about crazy people is that they're going to be crazy.