Walking turns into crawling

When walking around (i experienced this in the nether with the new crimson vines, when I walk into them, I temporarily go into crawl mode and then stand up. I also had it occur when there was no vines, also in the nether, and I walked into a block at head level but there was a one block space beneath it and it put me into crawl mode under that block in the one block space, simply by running into it