Watch your stems for signs of leaning. If they start straight up and down and after your first ride they are leaning then you are not putting enough air into them and the tire is slipping on the rim. When this happen and it puts enough pressure on the rubber then it'll tear. But just in case it's cutting through you could use a file or small knife blade to chamfer the stem hole's edge on the inside.

Something else you can do to check for tire creeping on the rim is to use a felt pen or some easy to remove paint and mark a line on the rim to the tire. If the part on the tire isn't lined up with the part on the rim after a few rides then again you have proof that the tire is moving on the rim and that is your trouble.

Also do you use anything to aid you in mounting the tires? Like a tire lube similar to what motorcycles and cars use to aid in seating the bead?