Police say the truck rolled down an embankment on Monday and ignited a blaze that nearly spread into the surrounding forest.
Crews were at the scene at about 5 p.m. extinguishing the fire and gathering evidence.
RCMP say the investigation is in its early stages and are not releasing any more information.
A section of Highway 99 was closed for hours after the rollover but has now partially reopened.
The truck remains at the scene and is expected to be recovered during daylight hours on Tuesday.