Budgies attacking each other

Question
I have two budgies, one male, one female. The female is older, about 2-3, I think, and the male is about 1 and a half.
They've been living in the same cage for about half a year without fighting, they always have food and water available.
Today, my male bird attacked the female, I wasn't there when it happened, but she had blood all over her bottom area, beak and foot when I returned. He had a small cut on his foot. There was a puddle of blood in the small water-bowl of the mirror which I keep empty in favor of a bigger water-bowl, and blood on the perches and cage bars. She didn't need any vet attention and the bleeding clotted easily. I decided to separate them, but since the other cage is quite a bit smaller, I put it inside the other one, so that they wouldn't get used to living apart.

She originally lived in the cage alone, but didn't mind him joining her after they were properly introduced. What could have made them fight, and should I put them back together?

Answer
Misa,

There is really no telling what made the birds fight. The male could have been attempting to breed with the female, and the list goes on. I would not suggest putting them together unless you could supervise them while they are together. It is possible that this could occur again, only with worse results.

Best of luck