420 SEL, cracked rocker arm - advice needed.

My 420 SEL "died" the other day and I had it towed to a local repair shop.

This same shop had just recently done a timing belt/tensioner/guide change. According to them, it now has a cracked rocker arm. The shop wasn't clear if there might be more damage.

They stated the timing belt had not broke, and timing still set OK.
They also said they would "do the repairs under warranty" since they did the recent timing belt work which might have resulted in the damage.

Any thoughts on what might have happened?

And anything I should be concerned/ask about once they fix it?

Even though their agreeing to fix it under warranty, I must admit I'm nervous about what went wrong in the first place and will it happen again....