That's a sympathetic vibration. It means your guitar is especially resonate for that A. I wouldn't think you would want to do anything to your guitar to permanently decrease its resonance. Try moving away from other instruments that are playing all those As.
As a side note, The Beatles were the first to
record sympathetic resonance as an intended inclusion in a recorded song. On
I Feel Fine, John's guitar, an semi-acoustic Gibson with a pickup, started a sympathetic resonance when John set it against his amp. That developed into an increasing feedback. It was a studio accident that they duplicated and used in the opening of the song. One account suggested that it initially happened another way, that a plucked A from Paul's bass started John's A string going...
The principal here is the same. The A from your bandmates is just like the A from John's amp. Both physically excite the A string on the guitars in question, causing it to vibrate more and more.
The solution is also the same. To stop the feedback, John moved away from his amp. To stop yours, move away from the other instruments for the songs where it happens.