There were two reasons for the problem: your movement code plus a bug in c_move.

The bug in c_move causes entities to get entangled on 'hooks' in some cases, in your case on the vertical surfaces of the triangle-shaped blocks you've made the 'hill' from. This special case will be fixed in the next version, but there can still be other sorts of hooks in the ground where the same happens.

For this reason, the real fix is a better movement code, as suggested by Damocles. NEVER move an entity by dragging its bounding box along the ground. ALWAYS keep a distance from the ground. Use c-trace to let the entity float in step-size height above the ground. The reason is not only the above problem, but also that movement by dragging is very slow because of the permanent collisions. If you have 1000 actors moving around, you'd end up with 10 fps with this method of movement.

Check the movement code in the templates.