When a crosswind hits an aircraft it affects the windward side more because the fuselage shadows the leeward wing. Therefore, what happens is that the wind tends to lift up the wing on the windward side. This only happens when the plane is on the ground because what happens is that the wind fills the space between the ground and the wing and pressurizes this area. This pressurization is called ground effect. If the wind is a headwind (such as when landing), then the area under both wings is equally pressurized, but if there is a crosswind, then the windward side will be pressurized harder and tend to lift up the wing. In such cases the pilot needs to use the aelirons to counteract that effect.
Note that any upgust can have a similar effect. For example, when landing on a windy day, the wind will hit the ground and move upwards creating upgusts. These can hit your wing and flip you over, if the aircraft is light. Therefore, when landing a pilot will always be wing down to the wind.