One, Ken Calvert is one of those scandal-plagued Inland Empire GOPers, like his neighbors Jerry Lewis and Gary Miller. It's mostly penny ante corruption, but he was also picked up with a prostitute in his car back when he was a freshman rep in the mid 90s. So although nothing new came out about him lately, maybe there's some cumulative distaste with him.
And two, this is the second fastest growing district in California population-wise. The majority of that growth is Hispanic. I don't know how many of them are actual voters, but I suspect the number of Democrats in the district is going up sharply. Riverside County in general went for Obama, and I don't think it's gone Dem since 1992.