It happens to all the airlines, every now and then. Here's UA's latest snafu:
http://www.flyertalk.com/forum/unite...-days-pvg.html
Looks like it was the last AS flight out of KOA that day. Which makes it more difficult to get put onto an SJC or PDX or SEA alternative AS flight.
I'm glad they were able to move you to the UA flight.