The Tour of Flanders if famous for its steep, cobbled climbs. If you have ever climbed a hill paved with cobblestones or bricks, you'd sit down and have your hands on the tops too. Having your hands on the tops puts more weight on the rear wheel, important for getting more traction, especially if the roads are wet. It's also more comfortable when the bike is hammering along on the rough roads.

Typically, you'd get on the brake hoods if you're out of the saddle.

L.