I may be able to help here. If memory serves me, Sirius and XM downlink in the Ku-Band satellite frequency band. In North America this is generally 11.7-12.2 Ghz. What may very well be happening is one of a few things.
1. Harmonic interference
When a signal is broadcast especially using high powered signals, harmonics are also broadcast. A cellphone tower is generally broadcasting in the 800 Mhz range, putting harmonics at 1.6Ghz, 3.2Ghz, 6.4Ghz, 12.8Ghz. Yes that is outside of the Ku-Band Rx range but with a high enough carrier even a 4th harmonic can be taking up so much bandwidth that it could affect the Rx only signal of a satellite radio reciever.
2. Broadband Noise
The cell tower may not be filtered enough and by amplifying the cellular signals, they are also amplifying a lot of noise (Gaussian White Noise) acting as a jammer of sorts.
3. Command and Control
While aiming a satellite terminal, I crossed a cell tower and picked up what seemed to be a carrier in the Ku-Band Rx freq that I figured (possibly mistakenly some cell guy will probably chime in telling me I'm wrong) was a command and control link between that cell tower and another or perhaps another location.