Wake turbulence.

Bottom line

All aircraft generate wake turbulence during flight, caused by a pair of counter-rotating vortices trailing from the wingtips. Vortex strength increases with an increase in aircraft operating weight or a decrease in aircraft speed; the greatest vortex strength occurs when the generating aircraft is heavy, slow, and clean. If an immediate takeoff behind a large heavy aircraft is necessary, the pilot should plan to minimize the chances of flying through its wake turbulence by avoiding the other aircraft's flightpath.