It has to do with the starting point of the bridges. That's the point the bridge loads in from. I'm trying to find a solution to this as well. For example placing a bridge that runs from the North Trop Island to the South Trop Island on The Center map doesn't work. I've had to line up a bridge on one side, then go place a bridge that lines up on the other side to get them to connect. It's a pain in the ass and I'm sure there's a way to keep the bridges from unloading.