pukes in crate

My JR was outside yesterday and he ate a bunch of grass which is nothing new to me, I have seen many cats and dogs eat grass. We went back inside and he was fine then all of a sudden he started gagging like he was going to throw up, again not too worried because I know most animals throw up after they eat grass. The thing is he ran into his crate as though he intended to puke on his bed. My question is why would he want to puke on his bed when I know he would never dream of soiling it in any other way? Luckily I coaxed him out and got him outside before he did, but I am still confused. Any insight would be great.