It's probably because of adrenaline.
I had this problem when I played in a basketball league at night. We played very competitively, so I'd get amped up, revved up.
And had a lot of trouble getting to sleep when I got home. Another guy in the league described the same thing happening to him.
Once you get a lot of adrenaline pumped into your bloodstream, it takes a while for it to work out of your system.
Adrenaline is basically like a natural form of speed. So your heart rate etc. is increased etc., and it takes a while to wind down.
When doing heavy lifting, you are probably getting more revved up for the challenge of lifting the heavy weights, so you probably get more adrenaline in your system than if you are doing light lifting or cardio.
So you may want to adjust your schedule. Do the heavy lifting at a time that is not close to your bed time, and not on a night before you have to go to work the next day.