This ebbing and flowing of symptoms is characteristic of hypertensive pregnancies. The way it's been explained to me is that we have "decompensating systems" -- which means the negotiation between mother and placenta is sometimes stabilizing and sometimes getting worse, and that when it is getting worse, we have some body system that is more sensitive and more prone to break than the other ones given the unique chemicals being produced by this placenta and our underlying susceptibilities.
Put another way, when the disease gets a little worse and the setup is for our livers to get hit first, we could start experiencing symptoms of HELLP acting on the liver (including the pain), and then they could go away for a while if the mother and the placenta reached a temporary agreement. Or if we're chronic hypertensives, our pressures will get spiky. Or if we've got underlying kidney issues, we could spill more or less protein. Or migraines and visual disturbances, or whatever.
Some of the researchers working on this really like the analogy of an arms race, where no one *wants* the all-out thermonuclear war, but sometimes the diplomacy just doesn't work.