My housetrained dog peed on the floor

Question
I had my brother and his fiancee over for dinner with my husband and I the other night.  We have 2 dogs, one 5 months old, and the other 4 years old, both rescue dogs.  I got lucky with the older one, he was completely trained when I pulled him from the shelter.  Last night, as we were eating, he just lifted his leg and peed everywhere, even though my husband had just taken him out 2 hours prior.  He has been stuck in the house for over 4 hours at a time, and has never done that before.  Why did he do that?

Answer
It could be one of two things

First of all it may be a medical problem such as a bladder infection or hypothyroidism,so it would be a good idea to have a vet look at him first

If that not the problem it may be because he may feel like he has to show the puppy that the house is his-now what you can do for that is to spend extra time with the older dog,so he doesn't feel left out,like talking him for walks on his own and playing with him on his own