Because it nearly turned into a war in OTL. The only reason it didn't is because the French were lukewarm about their support for the Spanish and the Spanish were unwilling to jump into another war with Britain without them, so they backed down. Remember the Spanish had just lost Florida to Britain in the Seven Years' War, so they were spoiling for a fight to get it back (in OTL they conquered it back in the American Revolutionary War).
Well yes, but I think the Spanish have more of an incentive to turn it into a war than the British. I'm not sure that California becoming a bit more likely and having some gold will make the British belligerent too - my idea of the British taking California was, as suggested, to be part of a war which was already in effect.
I guess it could happen though. I'm not saying I disagree with the idea entirely, I'm just not being hasty about saying that an early Gold Rush makes war far more likely.