What's disturbed the Gulls?

Walking Jonty and Foxy (yes, Foxy came too) I actually took the trouble to count the Gulls on the playing field. 49 Gulls, all facing the same way, odd one would fly up, around and land again, but they were all on the ground when they suddenly rose up almost all at once.
So, what spooked them? was it Jonty or Foxy? Another dog running across the field?
Extra photo shows the answer, it's the man that sets up the Saturday/Sunday childrens' football and he's got a new bottle of white line marking stuff.