First thing to check is to make sure he doesn't have any parasites making his bowels irregular. A fresh stool sample taken to the vet for analysis should help you determine that.
If he is showing any other unusual signs then you may want to have the vet do a full exam to make sure this isn't an illness issue.
Some dogs do this from anxiety about you being gone. Its not spite its more like that awful feeling you can get in your stomach if nervous and in response dogs just 'go' to help reduce the feeling.
You can read about separation anxiety and combatting it here
www.ddfl.org/behavior/separtn.htm
I also suggest increasing his daily directed exercise - I find a tired dog is a good dog and in many cases a longer walk or run, some extra games of fetch, some training time etc reduces the dog's energy for stressing.
Hope this helps you!
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