Maybe it is because dogs prefer to go to the loo as far away from their sleeping quarters as possible and so they go up to your room to do it rather than do it nearer to where they sleep. Maybe creating some type of barrier to stop them entering your bedroom?

It's a good idea to not allow dogs to have the run of the whole house anyway so confine them to certain parts. Also, to reduce their need to go in the house, let them out last thing to go outside and then first thing in the morning. As adults, they should be able to hold themselves for the night by now. Teach them to go at specific times by noting when they most likely need to go and use a command word that they learn to associate with relieving themselves and, when they do it in the right place, you praise them good and proper and this reinforces the behaviour and they are more likely to use that spot again in future. Dogs are creatures of habit and they have developed a habit of using your rug so you need to change it to something else and using the method described, you should be successful.

The rug in your room probably looked like a good substitute for the newspaper they have been trained to use.