Dog Pees When Left Alone in the House?

I am currently taking care of a Border collie puppy named Tang Yuan (it means dumpling soup in Chinese) who is about 7 months old. I had left something in my car that is parked right in front of my apartment so I told him to stay and wait and that I'd be back in 2 minutes and once I came back he had made a big puddle on the apartment floor.

The second time I had to go back briefly (another less than 2 minutes) he did it again, but thankfully I had the puppy pad there in the same spot where he made the previous accident. Is he doing that because of his anxiety or is it because he is excited? I hope someone can help answer my question. I am just curious to know and always open to learning something new about dogs. Thank you.