Can someone explain this phenomena?
Parakeets are social animals that live in large groups (flocks). Under ideal environmental conditions, they can form flocks composed of thousands of birds. http://www.softschools.com/facts/animals/parakeet_facts/1274/

The fact that this tree is without leaves is irrelevant to the Parakeets. They may or may not have caused it. They chose the tree as a place to relax. It could have been any other convenient place though. If you search google images for "parakeet flock" you will see them perching on various other surfaces in a similar manner