Trapdoor Glitch/Bug

I was just making a minecart tour(when you make a railroad and put a minecart on it and you tour around) and I noticed something. I had put a row of trapdoors on the side of my house and had them shut. But I saw that a lot if them were open. So I just closed them all. But then after about 5-10 seconds, I looked back and I saw that the a few of the trapdoors were open again. Then, I turned around and put my volume on and I heard the trapdoors open by themselves! I don't know why the keep opening but they do.