Unless your essays and recommendations raise several red flags, you’ll get into schools much more selective than UIUC. Your profile is strong enough for several of the top public schools on your list, and as a California resident you should have little trouble getting into most of the UCs.
I suspect UIUC made a similar assessment, assumed you were using them as a safety, and rejected you to keep their yield high. There’s even a name for this phenomenon: Tufts Syndrome.
I’m also speaking from personal experience. My first rejection letter came from a school that I considered a low match. It arrived a week after my acceptance to a reach school that’s everything the former hopes to become.