Vancouver city staff say a large tree that crashed to the ground in downtown Vancouver yesterday trapping two men in their vehicles, fell over because its roots were rotten.

The large maple tree unexpectedly fell on several vehicles at the corner of Cambie and Hastings streets Tuesday afternoon, trapping two men in a car and truck under a trolley bus line.

There was no strong wind or other apparent cause at the time.

The two men were freed by first responders after about an hour and a half, once BC Hydro cut power to the downed bus lines.

On Wednesday the city issued a notice saying parks staff attended the park last night to do a post incident assessment.

"They have determined the tree failure was caused by internal white crown rot, eating away at the base of the tree which weakened its root support."

The city said the collapse of the tree was highly unusual but staff will inspect the other large trees in the park to determine their structural integrity.

"According to the last safety inspection by inspectors in December 2013, there were no significant visible defects on this tree or other indicators of problems. The last photo from March 2013 shows a healthy looking tree," said the city.

The city says urban forestry crews carry out hazard tree inspections every year for large street and park trees across the city. There are more than 140,000 street trees and an estimated 300,000 park trees in Vancouver.