Broken nipples can be caused by too-short spokes. See the illustration; if the threaded spoke extends all the way to the end of the nipple, the cone shaped end is under compression, supported by the spoke threads. If the spoke doesn't reach that far, it's under tension and can break in half.

See the discussion here.



You can inspect the nipples by removing the tire, taking off the rim strip, and checking the tops of the nipples inside the rim.

The spoke end should be up right to the screwdriver slot in the top of the nipple. If it's way down inside, the wheel was badly designed and could be a warranty fix.