Every aircraft forms vortices as a consequence of generating lift. And large aircraft, particularly at low speeds, in the landing configuration, form large vortices.
This video, shot on a foggy day at Birmingham, England, shows these vortices quite clearly.
Depending on the conditions, especially on days when the winds are mostly calm, these vortices can actually persist for as much as five minutes. Note during the video that there is a slight left to right crosswind, and the vortex from the left side of the aircraft drift across the center of the runway. While these examples dissipate fairly quickly, the danger is that one might come across a vortex from a previous aircraft while landing. And usually, pilots don’t have the visual cue of fog to clearly show the vortex.
More than one aircraft has been doomed by flying into such a vortex. These powerful disruptions have more than enough energy to flip over an airliner as large as a DC-9.
There’s a reason why air traffic control has to space out landings behind larger aircraft.