Do you use a microphone windshield? (a foam cover)
Many pilots do not, so as well as being difficult to be understood by ATC owing to audio distortion caused by plosive sounds, the microphone gradually fills up with crud. Aviation microphones have a passive noise cancelling feature which reduces cockpit background noise from the mic - as long as the mic is clean internally, has a windshield and is placed very close to your mouth. If crud builds up inside the mic, the noise cancelling feature stops working and your mic puts more and more background noise on your transmissions, which sound noisier and noisier.
This noise might be what is triggering the VOX of your intercom, (if fitted). If so, there might be an adjustment control of the VOX switching threshold on the intercom unit.