KLAC does not have a backup transmitter *site.*
Unless you know something I don't, there's more than one transmitter in the building. There certainly was the last time I was there, though it's been a few years.
I can't think of any iHeart station I've seen that doesn't have a backup transmitter, even their lowliest and most forgotten little outlying FMs. The company believes strongly in backups and planning.
So what happened? They are indeed under no obligation to tell us. Since co-located KFWB (same towers, separate transmitter building) apparently was not affected, we can say with certainty it wasn't the towers, and we can reasonably speculate it had something to do with the STL, but it could also have been anything else specific to KLAC's side of the site - a failure to switch over to a backup transmitter, a problem with the power handling in their specific building, an issue somewhere along the transmission line out to where the 570 and 980 signals are combined. Could be anything.