After watching and re-watching the episodes, I've come to the conclusion that Mumm-Ra is controlling Cheetara.
Think about it. Before Lion-O and Tygra go into the astral plane, Cheetara is constantly giving Lion-O signals, to the point that EVERYBODY thinks she likes him. Then Mumm-ra tries to get them to kill each other by preying on sibling rivalry. And once they return, Cheetara (The girl they've been fighting over this whole time) chooses Tygra despite all her signs.
In the next episode, she is perfectly okay with letting the lizards that defected from Mumm-Ra's army get punished and then drops her staff in the middle of a fight, completely dismissing all her training to protect the crown. Then you have two Character backstory episodes, and then in comes Pumyra right when Lion-O is vulnerable.
And finally, The sword of Plundar is sealed by Jaga's magic and everybody that tries to take it dies, EXCEPT Cheetara who just happens to volunteer to get the sword then Pumyra just happens to let it fall into Mumm-Ra's hands after being "trapped".