Hormone control. He sees her as a mate and so wants to mate with her. Tiels will mate for fun, so sometimes there are no eggs, but honestly it's best to stop it before it starts. http://talkcockatiels.com/showthread.php?t=32330
this sticky has information on using hormone control to keep it under wraps. This is a year round thing, not just something you do short term.
That being said, if it doesn't work they may need to be caged separately but right next to each other. That's always a possibility when keeping birds.
Thanks DallyTsuka for siggy!
Snowball-whiteface cinnamon cock
Jeep-normal gray cock
Pebbles-whiteface pearl pied hen
Pankakes-PF cinnamon pearl pied cock