Probably because you are using funds that aren't confirmed in the first place.
That may be caused because you don't know what you're doing or because you are using a bad wallet like blockchain.info
BAD?!?! -gasp- HOW COULD YOU?
---------
Yeah, I gotta admit that blockchain isn't the best wallet.
Blockchain, like many wallets, doesn't let you spend outputs whose inputs are not confirmed.
Supposing that you have collected this transaction from many smaller transactions, check the inputs for the transaction. I guarantee there will be unconfirmed imputs.