Because Umar has this ability to tag on to the ones who are getting a lot of public support. First Farah said and he clung on to KK/Teja and threw Afsana under the bus.
Then the way Rajiv was being mentioned in WKW that he has been saved and is getting a lot of support he started his work. He noticed he is a little vulnerable and dumb to go if he is shown some (fake) support. Plus the icing was that Umar used Rajiv by pumping him against Pratik who Umar hates anyhow. Many birds in one stone.
Acha then Umar is a mastermind right.
Umar never clung on Tejran. He survived in the show by his own. UmRan became friends during the captaincy task won by Umar and that was not in second or third week when farah came.
UmTej are still not great friends so how can umar used teja.
Umar threw Afsana under the bus acha don't forget it's umar only who helped her in next task. And now afsana calls him her good friend.
Rajiv and Umar are friends before this wkw. Umar is not using rajiv but rajiv is using umar. Umar does not need anyone to survive in the show