I think it's just their theme(like the spanish hollow and arrancar stuff), I think it started with Uryu's seele schneider, which is german too and means soul cutter
btw, the Bounto also had some german words to say, the funniest part must have been when one Bounto made a doll in iPod-Bounto's flashback
Kariya did very well though when he released his power using a german spell right before the final fight, a bit rammstein-esque but not bad