What's causing this pothole?

Enbridge put in a line recently on a major road, an 80km single lane highway with fairly busy traffic. They opened up a 1ft by 1ft section of the road that has continually resulted in a reoccuring pothole immediately after the hole gets patched. The pothole has been filled at least every week, sometimes twice, thrice a week since the line went in. A pothole forms again 2 or 3 days later without fail.

Any best guesses why the pothole keeps opening back up?
Gas leak?
Water leak?
Improper patch?
Too much traffic to properly secure patch?

Patches usually last a month or so before they open up again. This one persistantly opens back up within days of being patched every single time. I suspect there's some kind of leak and the hole will be permanent unless that ever gets addressed. However I'm not an engineer, but I'm curious if anybody knows why some holes cannot be patched in the interim.