"Dirty Air" as stated above, same thing happens to big helicopters...
Also known as Settling with power, this is a condition where the helicopter begins to settle in its own downwash.
The general cause of this condition will likely be a combination of high gross weight while landing in a down wind condition. Although VRS can occur while landing into the wind, it is not nearly as common.
There are three conditions required to be present to get into settling with power, I call this the 1,2,3, rule. You must have airspeed less than ETL (many teach 10-knots or less airspeed), you must be using 20-percent or greater power, and you must have a sink rate of 300-feet per minute or greater. These are minimums; I teach my students to maintain not less than 30-knots in the primary descent. Most importantly, the sink rate must be reduced to less than 300 fpm before airspeed is reduced to less than 30-knots.
Awareness requires that pilots know their gross weight especially where the power will be limited, and they must also be very aware of the direction of the wind. Never should a helicopter be landed in a downwind approach that will terminate in the downwind. If a downwind approach is necessary, the airspeed should be maintained above 30-knots (a good safety margin) until the helicopter is turned into the wind, and the sink rate is reduced.
Settling with power will be recognized by a vibration of the main rotor system, quickly followed by a rapidly increasing sink rate that can reach 6000 fpm. You don't want to be close to the ground when that happens!
The recovery procedure will be to input forward cyclic to fly free of the vortices, and simultaneously decrease the collective pitch reducing the size of the vortices (decrease power).
Avoidance requires that you remember that all three of the conditions stated above in the 1-2-3 rule must be present, so you only need to keep one of those conditions absent to avoid that phenomenon, and consequently you need only eliminate one to get out or the phenomenon of settling with power (VRS).
It is also important to note that although teaching that 10-knots is all that is necessary to avoid settling with power is correct by most published information, I feel that it is not sufficient airspeed to safely avoid the condition. If you are flying at 10-knots, all you need is a slight wind shear to put you in the condition, and if that occurs near the ground, look out.
Always be cautious.