What Kerisue said.
Both HELLP and placental abruption are thought to stem from initial implantation difficulties for the placenta and/or ongoing difficulty tolerating the placenta during pregnancy, mostly because the placenta is a foreign organ. The maternal immune system usually tolerates the placenta even though it's foreign, mostly because it hides, but sometimes the ability of the placenta to coax the maternal immune system into ignoring it breaks down/ doesn't work so well, and these hypertensive diseases of pregnancy are the downstream result.
There's a bunch on this in the "Announcements and PE in the News" section of the forum and some posts to the research blog on the main page as well, with more blogging to come (in just a few days that blog will become a steadily active part of the site, but it's been one of the slowest to come online after the site overhaul. Which is my fault, lol.)