Was it because:
1.) It was retaliation for all of the throw rug jokes.
2.) Because he feared Zazu might recall being hit by him right before Mufasa's death.
3.) So Zazu couldn't leave (Zazu had made some comment about leaving if Simba, on his current course, became king. He might have actually left when Scar became king and brought in the hyenas, and perhaps Scar didn't want him to leave.)
4.) Merely so that Pumbaa could rescue him later.