There is a bit more to it than that. He was locked into Cal and was set to sign and supposedly, Cal's current QB called him the night before and talked shit. Pissed off, he surprised everyone, including Oregon coaches, by signing with Oregon the next morning. Mendoza then promptly leaves Cal (why the fan base hates his guts now- leave if you want but fuck you for sabotaging on your way out) and JKS is in Oregon. Now Mendoza is gone and Cal has been backchannelling to have him come in anyways as a transfer. Oregon is spinning it that he saw the competition in Eugene and is leaving for Cal because he will start but Cal is getting a 2nd portal QB because they only have 2 QBs on scholarship currently and they both suck so it is not 100% guaranteed they hand him the ball day 1 (Devin Brown is being whispered about as the other guy). Did he see the Oregon QB room and decide to leave? Maybe. But Cal is "supposedly" where he wanted to go so its not a huge surprise if he goes back.
All allegedly, of course.