I've known gardens have a lot of rats around the bird feeding stations seemingly without any effect on the birds, so I'm not sure that would be the reason. However, in my own garden I happened to see a sparrowhawk attack a large flock of starlings, taking a juvenile, and it was a couple of months before a few cautiously reappeared, and they've still not returned in the numbers they had previously, so perhaps it could be a predator has been seen and spooked them.
Didn't one of the BBC Springwatch programs feature a flock of starlings settling under Aberystwyth Pier after an evenings murmuration, only to huddle down and become lunch for one BoP, which had patiently perched and waited amid the starlings nighttime roost?
Mike
Flickr: Peak Rambler