Hi, pilgrim, fear of tight spaces can be the symptom of a subconscious trauma in another tight channel:
The birth channel of the mama dog, and a beginning struggle for life in those puppies who are born last.
Oxygen may become less, and the contractions of the womb can become increasingly stressful for those who are not the first ones to be born.
I hope that is a helpful explanation.
Of course it could be something else as well.
I even read that pets often mirror the issues the human owners have, but of course I can't say he is a mirror of YOUR issues, I don't know you. But perhaps that is also a helpful information.
A very young puppy may even have been uncomfortably stuck between the cushions of a couch, and struggled for air, and now be suspicious.