Originally Posted by Iride01

The part on the end of the pump's hose that you attach to the valve stem to fill it with air.

Many on floor pumps have a clamp lever on the air chuck to hold them onto the valve. If the clamp lever doesn't release fully or something else happens to the air chuck might be harder to pull off and might easily be enough to strain the boot on many a presta valve tube. Perhaps Schrader valves too.

Do you fill your tires with air from a compressor or something that just has an air chuck that you hold on the Schrader valve? Perhaps you push the valve stem down in the rim as you fill with air and the boot is getting damaged from that. Especially if you are bottoming out the stem on the tire. If you are pushing the stem in as you fill the tire, don't. I just used my free hand to lay a finger on the stem and rim so as to wedge the valve so it didn't get pushed in.