All three districts are around 45-48% Democratic and 23-28% Republican. So they're districts no Republican could win. You have to be a Democrat. Bill Bloomfield tested that as an independent but couldn't pull it off. He likely couldn't get enough Democratic votes. Swalwell was well positioned. He was a Democrat Democrats could vote for, but also the not Pete Stark that Republicans and independents could get behind. Negrete McLeod's victory surprised me, due to her vote on HSR. Baca didn't make it an issue and Michael Bloomberg's PAC was able to sell Negrete McLeod as a moderate.
The lesson of 2012 is that a Democrat running against a fellow Democrat can win as long as he or she proves to Republicans and independents that he's the more moderate choice. The Democrats has to get a decent level of the Democratic vote, probably 30%, but they can clean up with the other half of the electorate.
There are plenty of other districts where this could happen. A Democratic challenger needs to make top two first, of course, and that's not easy if there's a Republican challenger. In the CA-32 primary Republican David Miller got 41.8% of the vote and Democratic challenger Bill Gonzalez got 12.1%. Swalwell and Negrete McLeod were helped by no Republican entering the race. If the challenger can get past the primary, they can beat an unbeatable entrenched Democrat is they can position themselves as the alternative for Republicans and independents.