Sudden aggression

Question
I have a almost 3 year old great dane (Mack).  I also have a 8 year old
cocker spaniel (Toby). They have always gotten along really well. They are
both males and both are neutered. Mack has been the best dog. Very well
behaved and never aggressive. Today he got into a fight with Toby. He
had Tobys head in his mouth. I broke them up and separated them. Toby
did get puncture wounds, but he is ok.  Any idea why he would be that
aggressive towards one of his own family members? And do I need to
worry that it will happen again?

Answer
Mack is a Great Dane and Toby is a Cocker Spaniel, so there is a significant size difference.

Do you know for 100% certainty that he was being aggressive, or was he playing hard and accidentally harmed the Cocker?

If the bite was actual aggression, yes, there is a chance that it can happen again.

But I don't know the circumstances of the fight. I don't know if it was provoked, if you saw the fight. All these details are very important.