Aftermarket springs are lower that stock, they bring stock struts out of their designed base level position, this is minor though.
A stiffer spring has a faster oscillation rate than the OEM one. Hit a bump that compresses the spring 2" and it will bounce back a lot quicker that the stock sping. That faster spring movement makes the strut shaft travel much faster through the body. This can lead to foaming of the oil within the body, as the valving was not designed for those shaft travel speeds to begin with.
This is what people refer to as a "bouncy" or "harsh" suspension setup. There is a bit of aftershock because the damper cannot dampen 100% of the spring's force in a single pass. The spring over-entends so the strut has to have a second take at damping it. A lot more movement there, faster than designed, away from the designed "neutral" position, etc..
I think all these factors are what's taking a toll on the struts. The high speed movement of the damper's shaft puts a lot of wear and tear on the valves. They wear out much faster, letting more oil go by, damping less, letting the setup move even more, etc, etc.. vicious circle that destroys everything within a few months of hard driving.
