I've sometimes had that happen, it usually seems to be for one or more
of these 3 reasons:
1. Worn jar sealing ring[1] (equivalent to not having lids screwed on
tightly enough);
2. Brought to the boil too quickly;
3. Boiling too vigorously, so that the jars rattle and knock together.
BTH
[1] I use Vacola[2] jars rather than Ball ones.
[2] http://www.bakeandbrew.com.au/category39_1.htm