A very, very smart veterinarian  who knows a lot about this drug after decades in the business of treating dogs reports that dogs who are given this drug often just want to go to sleep- especially when they are already tired.

Some basic research about the side effects of tramadol on dogs shows these side effects:

"Like most medications, tramadol has side effects. Most dogs tolerate tramadol well, as long as the dosage instructions are followed properly, but there are a few adverse reactions you should be aware of."

Vomiting
Nausea
Loss of appetite
Constipation
Diarrhea
Drowsiness
Anxiety
Dizziness
Tremors

http://www.akc.org/content/health/articles/tramadol-for-dogs/

This is NOT something a musher is going to give his dogs at the end of a long race- unless he is trying to lose.

It is extremely likely someone slipped this med to his dogs after the race, maybe mixed in with some ground beef.