What causes these weird physics on models?

Go and spawn a combine thumper prop. The path is models/prop_combine/combinethumper001a.mdl from HL2.

Pick it up with your physgun, and try to fling it across the map. You’ll find that you can’t - if you try, the prop stops moving as soon as you let go of it, and slowly falls to the ground.

Both types of thumpers do this. There may be other built-in models that do this, but I haven’t found them.

Does anyone know what causes this? Perhaps some missing physics information? The reason I’m asking is that my new model also has this behaviour.

Read about some Collision QC commands. You might want to mess around with some of these to get rid of this physics effect you are describing.

My guess: it appears $rotdamping/$damping can cause this (according to valve dev wiki badly configured $rotdamping might cause that effect).

All of the values make a difference in some way, including mass and inertia as well. The thumpers never move in HL2 so they were probably given arbitrary values.

Isn’t studiomdl supposed to calculate those values by default?

Yes and no. If there’s nothing it bases it off the volume. Which means absolutely nothing in the longrun.