Bridges are not the only surfaces that can make the bike feel like it is on ice, losing traction or slipping laterally. Most roads regardless of how smooth they look, have longitudinal grooves. Some wide, some small. Depending on what part of the country or what part of the world you live in, you will see different types of road surfaces and conditions. Because the Ninja has narrow tires it is easy for it to do what some call tramming or tramlining. That is where the tire rides in and up or up and over a groove, giving the feeling like the rear or whole vehicle is moving from side to side. It's like going over a grated bridge, you get the side to side movement because the tires are riding on grooves in the metal. Add in the crown of the road, tar patches, sand, loose pavement, water, antifreeze, oil, transmission fluid, rubber residue, and whatever else rolls across the road, there is no telling what is the actual cause of lateral slipping. The remedy? Be more observant of the road surface your are riding on, take the appropriate action for the riding condition, moderate your speed, use even braking if you need to, use appropriate steering measures, maintain good control over the bike.
If you happen to get caught in some crosswinds, there are a couple of things that you can do to make the experience less stressful, however. For example, you can quit white-knuckling your grips. When you hold on tight you also tend to stiff arm your controls. That, as we've talked about before, merely allows front-end instability to propagate into the rest of the motorcycle. Relax your grips and droop your elbows. Allow your bike to be a bit unstable. Drive in the center of your lane. Lean forward and down to reduce your profile, snug up your jacket. There will be a couple of times when a gusty crosswind changes your bike's direction of travel: when it hits, and when it stops. Both require that you allow the bike to respond and use normal modest counter-steering. (When it quits you will be leaned over and, as a result, your bike will move towards where the wind WAS coming from until you straighten it up.
If crosswinds involve huge short gusts, pull over, park the bike. If the crosswinds are more sustained, pucker up and keep going then...just lean wit it, rock wit it, roll wit it!!