it's caused by a faulty pcv valve or blocked tube/breather box.  the
collapsed tube itself is no big deal unless it's lost its rubberiness
and won't spring back to shape when the vacuum's gone.

Depending on the cause, excessive pressure in the crankcase, and/or oil in
the air intake pipe to the throttle body.

Does your PCV valve connect to a hose that goes down the back of the
engine, through the intake manifold web? If so, the oil separator box may
be sludged up. If your PCV valve is the sort that connects to the top of
the valve cover, then the valve cover baffles may be sludged.

Check for oil in the intake hose and the throttle body.

You may simply be using an inadequate PCV hose, such as an aftermarket one.
Is the PCV valve aftermarket? Aftermarket PCV valves can have incorrect
flow rates that can cause other problems.