We miscalculated the amount of sail we would need. The original designs didn't have the variably-reflective layer, and when we added it the sail gained mass which we forgot to account for.
Are you trying to tell me that you didn't check your mass before you left on an interstellar voyage?
Would that have caught the problem?
You know, for advanced intelligences, you're really, really stupid.
Hey, we're not the ones with meat-creatures crawling all over our ship.