Pilot error caused by a steep bank at low level caused the crash. Both pilots survived the crash.
Here are some details from Livefist
http://livefist.blogspot.com/2009/10/dhruv-crash-update-5...
"...the ill-fated helicopter is likely to have come under the air forced downward by the rotors (rotorwash) of one of the other Dhruv's flying in the formation. At this point, complications may have started when the pilots Luis Armas and Ivan Abril made an attempt to recover from the ensuing sharp left bank (this is visible in the video). In technical terms, the pilot encountered a cyclic limit to the right (saturation of cyclic - the stick for lateral movement of the helicopter), as a result of which they found they had no further cyclic available at their disposal to stabilise or roll back out of the left turn. In well-documented helicopter flight dynamics, when cyclic saturation is reached, there is an abrupt loss of available lift to counter the turn.

It is understood that the Dhruv flight manual and training programme contains a specific module on recovering from a cyclic limit situation, and the Ecuadorian pilots underwent this process as part of their training in 2008. The training however, taught them how to recover from this situation when the chopper's altitude was at least 1,000 feet. Therefore, considering that the Dhruvs were flying at just 70-meters above the ground -- and if this theory holds -- then the pilots would not have had a chance to recover either way." (less info)