Puppy licking the wall through the bars of his crate

I...really wasn't sure what category to put this in, so forgive me if I'm in the wrong section. My puppy has been very good in the crate so far. No crying, whimpering, or fussing at bedtime. He goes right in and sets to work on his Kong treat, which he is ONLY allowed to have at bedtime in the crate, or a rope toy. Tonight I noticed that he was bumping around in his crate, making a lot of noise. I looked over and saw that he had his face crammed against the bars near the bottom of his cage licking something. Upon closer inspection, it turns out that he's been licking the wall so badly that he took the paint off. What the heck?! I moved the crate away from the wall and he settled back down without a sound...why on Earth would he be doing that?