Placing redstone dust on a block closes opened trapdoors attached to it

I was making a mob farm when I encountered this problem.

I put a line of iron blocks and attached trapdoors next to it. I repeated it for several hundred times and then opened all the trapdoors. Then I decided to add a simple switch that would turn on/off the farm by redstone lamps. But when I placed redstone dust on top of iron blocks, the trapdoors attached to them magically closed. (The redstone dust is unpowered)

I think this can be caused by a redstone update on the iron block: the game updates the status of the block and thinks that the redstone wire had just been switched from powered to unpowered, therefore closing the trapdoor.