Yeah, it's a progressive problem that shows itself suddenly...
Over time the PCV goes bad, rings deteriorate making excess crank pressure, then the rubber on the dipstick wears and weakens with heat and time and mods increase boost pressure which increases crank pressure and then eventually the pressure/retention ratio crosses a certain point and the dipstick pops out.
Pull the dipstick out, crimp it a little with a pliers and put the dipstick back in, do this until it feels tight but will still move in and out without major difficulty.
Alternatively... you can build some kind of auxillary retention device. A clutch style spring, only alot weaker, tied to the dipstick tube with a small hose clamp, so that you hook the hooked end of the spring into the dipstick loop would work. Or something similar.