<background vortex information>
When fluid flows over an aerofoil as with air over an aeroplane's wings, a phenomenon known as vortex shedding occurs. This can be observed (in a wind tunnel) as a series of vortices coming off the trailing edge of the wing. A vortex will appear in a clockwise direction from the upper surface, then in an anticlockwise direction from the lower surface, and this cycle repeats itself. Vortex shedding caused the failure of the famous Tacoma Narrows bridge, - the frequency of vortex shedding matched the natural frequency of the bridge.
Now, to answer your question. The issue of "vortex breakdown" involves the amount of time it takes for these vortices to settle down after an aircraft has passed through the air. If another aircraft passes through the same air before the vortices have subsided the following aircraft can experience severe turbulations. This is the reason that ATC allow a set time between aircraft taking off on the same runway (and not for collision avoidance!).