What can cause a crossover tube housing to leak?

Yep, take it back to the dealer and be nice, explain what you did here. If they still insist the leak was not caused by the work they did, disturbing the crossover hose, then get firm. tell them there were no leaks prior to 'your work"... then if they still won't fix it RIGHT ask to talk to a supervisor, hold receipt for what you paid up and make demands..not throw a fit, just firmly demand satisfaction. Hopefully there will be other customers within earshot...that always puts them on the spot.

I'm the original owner. The cooling system has been flushed before and I have not had any prior problem.