Bruce pretty much summed it up - there are a number of ways that a phone can be active when you think it is idle, and a number of paths that interference can sneak into your signal chain. And you never know when it's going to happen, unfortunately. The only way to eliminate it is to have you and also everyone else near your equipment put their phones into airplane mode or power them down. Good luck with that!
Case in point: I have a set of powered KRK monitors at my desk connected by balanced cables to my Focusrite Saffire external FireWire interface. The fact that I'm using FireWire, with a dedicated FW card (much more crap going on in your computer on USB), to a fully metal-enclosed interface box, with balanced cables to shielded monitors would make you think that I never get cell phone noises. Not the case. Sometimes when I have my phone on the desk next to my keyboard, there is the telltale noise in my right monitor as my phone is doing whatever it does on the network.
Even well-made balanced cables can sometimes let in RFI, sometimes the connectors are at fault. Neutrik makes the
EMC series XLR connectors specifically for this reason.
Again, your 853s have a long unbalanced cable run, with another (very short) unbalanced run from the UBB to the M10. So it's not at all surprising this happened, even though it may seem very random. I think you've been very lucky that this is your first RFI incident. I'm not suggesting that you have to go out and buy new things; just that it's one of those things that happen. All you can do is minimize the chances, but like my example above, it still may creep in.