I suspect tire dust and particulates in the air, the streaks are just the location where the light
dew, mist, or rain run down. Bet if you watched sides in a drizzle you would see a rivulet of water where you see the streaks. The water just concentrates the stuff that has settled on the roof.
That's my theory and I'm sticking to it, not that butyl tape or other sealants might not cause the same issue but I get streaks that run from the roof down. Guess it could be from roof vent butyl tape.
I have caused mine to change location by changing the angle of the trailer.