Haven't noticed this before the newest snapshot (20w15a) so I think it might be a new bug. I did search for existing tickets but found nothing quite matching this.

What happens is that I start to crawl when walking past blocks, specifically warped wart blocks in the nether (see attached video). Maybe it's only happened to me with that block type because they're so ubiquitous in the areas that I'm traversing frequently, and they're often floating in the air due to generation and/or enderman griefing.

Also, I can't say this definitively, but it seems to happen near the corners of blocks only, i.e. when I'm walking around a corner with a warped wart block on the inside at eye height, and an air block below that (at foot height).

It's happened to me about 5 times already today but I cannot reproduce it intentionally as much as I've tried.