How do you deal with sudden under the bed aggression?

Hi fellow members. So I've recently finished a 8 week program of Obedience training classes with my 6yr old Shih Tzu. Recently I've noticed that his behavior is very aggressive whenever he goes under the beds. He growls and barks, but has no objects he's protecting. He's absolutely fine when roaming around the house; appetite is normal; lessons we work on from class tends to have a good outcome; walks/playtime & even excise is also good as well as the environment. No changes with food or treats. He's hardly ever left alone, on occasion it's mostly two hours or less. I've also haven't ever crated him since I got him at two months of age. I literally have to bait him from under the bed with human food because when he gets in that aggressive mood his treats aren't worth coming from under the bed. What should I do, even if the outcome is to take him to the vet? For the problem I definitely know isn't a physical or health one.