[CJ421]

It's because you did not change the stock dampers. The stock dampers are designed to work only with the stock springs.
The car sits lower because the H&R spring is shorter (and stiffer) than OEM; the result of that is the dampers are more compressed and have less room to move up and down. That's called damper travel. There's not enough and you're bottoming out.

Solutions:
#1: Reinstall stock springs.
#2: Get dampers that will work with your H&R springs. Koni Yellow/Sport come to mind.
#3: Sell H&R springs and buy a coilover kit, which is a damper/spring combo with adjustable ride height.

Alignment required in all cases.

Cutting the bumpstops is NOT a solution and would be IMO a worse idea than leaving them as-is. Bumpstops act as a secondary spring and prevent the damper from bottoming out; while you would regain some damper travel, you'd also make situations where the dampers compress fully (which are far more frequent now that there's less travel) more jarring. The bumpstop is cushioning you right now.

I like solution #2 if you like the drop these springs gave you.