do you have evidence of this? Last report I read from FAA is that there was more than 1 in the tower at the time. And as for staffing Pearson tower was staffed by a single controller and an assistant for midnight shifts starting about 11 p.m. for decades. In order to get relief they would have had to call someone in so it is no surprise that it would take time to get relief in. Regardless, both CATCA in Canada and NATCA have been fighting this issue for decades trying to increase minimum staff in all units to cover just such eventualities. It has always come down to budget. As for causes, my educated guess regarding principle cause will focus on procedures regarding when Jazz received his landing clearance and the coordination between tower and ground if their were 2 on duty. I have had vehicles enter a runway without clearance but never had a ground controller put a vehicle on the runway with traffic cleared to land.