I didn't think that could happen with a digital signal, but a physical quirk called “intermodulation,” happens when two radio frequencies combine to create a third.
"SiriusXM’s signal problems were first detected last year when customers in New York City complained their radios went silent during their morning commute." If this only happens in large cities, I suspect the tall buildings blocking the satellite signals has more to do with the problem. It is not a new problem at all in NYC as I've seen this myself for many years (long before the AWS spectrum use) and I've always known the buildings are just blocking the satellite signals. When you drive away from tall structures SiriusXM works fine.
I think they also use terrestrial antennas/repeaters in cities. Could that be part of the problem? Found a picture of one in the gallery. The small up pointed dish mid-pole. Click for larger: