It's easy to hypothesize that there's some not-as-of-yet described force acting on the muons.
This is how the BBC presented it, but their article was super dumbed down. I watched PBS Spacetime last night and they didn't mention a "new" force at all.

What stuck me from PBS was that a muon is basically a "heavy" electron... it's identical to an electron in all ways except mass. I find that very interesting. My best guess is the muon interacts with the Higgs field differently to an electron. What makes a muon different to an electron? Why does the muon interact differently? Why is it otherwise identical?

I'm not going to pretend to have anywhere near as good an understanding of the SM than I do GR, so this is all way beyond my grasp. A new "force" seems a little unrealistic though. The BBC even referred to gravity as a fundamental force. But GR says gravity is an inertial force, aka a "fictitious" force. And that makes sense when you understand what GR says gravity is... spacetime curvature rather than particle interaction. So I'm not paying any attention to what the BBC say.

I'm not surprised we have found something that doesn't agree with the SM though. It's clear there is something missing, from both the SM and GR, because they don't work together. I hope we're moving in that direction... unification.