House Trained Dog Peeing and Pooping Inside?

We have 5 and 2 yr old Chihuahuas. The 5 yr old has been potty trained since he was a pup, same as the 2 yr old. We have had both since they were pups. About a month ago the 5yr old has been pooping and peeing in the kitchen at night.

We take them out all the time and they also have a doggie door. We also have a box inside that has real grass in it for when we are gone during the day. They both have used the inside box as well as the doggie door. Why would he start this now?

Thanks.

By Sherrie S from Placerville, CA