5 month old lab suddenly hates his crate again - is this normal??

Biscuit has been doing really well with his crate since we brought him home at 8 weeks. he'd whine the first couple nights, then settled into a routine, slept through the night by 12 weeks, no big issues. we felt like we'd cracked the code.

then two weeks ago, at right around 5 months, he just... stopped accepting it. puts the brakes on at the door, has to be basically lifted in, and then whines and scratches for 20-30 minutes after we close it. nothing has changed about the crate, the location, the bedding, or our schedule. he's been fine all day, gets plenty of exercise and play, so i dont think its an energy thing.

is this a phase? did we do something wrong or is this just what 5 months looks like? we're back to sleeping with one earbud in to catch when he stops and can let him out, which i thought we were done with. any advice from people who've been through this?