I'm really surprised if that is the case, but I'll bet it was because of the already weakened state of the lily. Round up just doesn't normally work that fast. I can't say I have ever applied RU on a lily, but I have with several other kinds of bulbs. It always takes 2+ applications, and sometimes just doesn't work period.
It's on my list now; sometime if I have RU to use up, I am going to try it.
