I can explain. Cellular used frequencies between 800 and 900 MHz. Digital cellular (PCS) uses higher frequencies around 1.9 GHz or 1900 MHz.
If you look at the frequencies used by garage door openers, their range is different than cellular or PCS. So, how can this happen? The answer is spurious emmissions. That is the cellular RF amplifier is emitting unwanted signals outside of the specifed range. This typically happens when amplifiers are overdriven or are overloaded.
The FCC provides guidelines for spurious emissions that the cellular provider must conform to. This is to prevent things like you are describing.
Now the dilemma. How to prove it. If you contant the FCC, they may come out and test, or they may not. You probable have an intermittent problem from an unkonwn source.
If the cellular amp was continuously putting out these emissions, you would have a continuous problem. Or it could be from a cellular channel that is infrequently used, or could be from another unidentified source. It could be some neighbor with a CB radio that doesn't work right.
It takes a receiving device with a spectrum analyzer to test for these emissions and they may only occur at infrequent times.
Some of the newer garage door openers work be transmitting a code with the RF. This way spurrious signals typically do not recreate the exact pattern that triggers opening. So you only real solution may be to replace the unit with a new one that is not succeptable to this problem.