Someone could have spilled something sticky on the road like oil or some kind of car fluids, soda (spills of that are more common than you'd think) etc., I run Conti Gatorskins and the odd little piece of gravel sticks, no problem.

You're going the same route, but I bet the other side of the road on the return trip, that says even more for a spill in a localized area.

The problem should go away after a while and especially after a good rain.