Why do some of my aircraft rocket off like they're being catapulted from a carrier after slewing? Even if I have engines off, they shoot off going like 100 mph instantly.

The elevation changes as you move around an airport. If FS determines you are off the ground, even a foot or two, it puts you into full flight. Before coming out of 'slew mode' hit the "A" key(down) to reset your aircraft on the ground. I even go so far as to follow up with the #5 on the num pad for full stop too. Hope this helps

Try "F1" Too

This value :static_cg_height=... in the aircraft.cfg file will be too low.