Usually when you collide with a block, the player stop or at least keep walking against it and slightly "slide" in the direction of the aim. This is the case also with the Anvil, but only on the short sides of it. Because when you walk against it on the long sides, you start vibrating and you wont "slide" to the sides as you do walking against other blocks. You get stuck. But you can back off from it, but not press yourself by it and go around it.