They broke up over strawberry jam.
Pansy came to breakfast late, as the elves stopped refilling the serving dishes, and asked for a piece of his toast. Because he was operating on exactly two hours of sleep after staying up almost all night working on that damn cabinet again, because he was stressed over how he would answer for the delay in his mission, because he had potions coming up first thing this morning and he was plotting how he could sneak out a few key ingredients from the supply closet unnoticed, he didn't think. He handed her the piece that was in his hand, which he had not bitten off, mind you, rather than the piece that was still on his plate.
The piece that was still on his plate was plain. The piece in his hand had been spread with strawberry jam.
To quote Seamus Finnigan, who had reported the incident to Gryffindor house, "She went nuclear, she did." Whatever that meant.
Pansy had thrown the offensive piece of bread at him, and it had stuck to his robes.
She shouted at him, in front of everyone like some muggle streetvendor, that she could not believe, after all this time, that he didn't remember she didn't like strawberry jam. That he was the worst boyfriend ever, because he didn't remember the little things about her. He had forgotten both their anniversary and her birthday this year. That he never spent time with her any more, he didn't play Quidditch any more, he didn't do anything fun any more. That she didn't know who it was, or where he was meeting her, but obviously Draco had a bit on the side. That Pansy could not believe that she had wasted a third of her life being his girlfriend just for him to treat her like this.
Draco sat in silence and let her rage. He couldn't even summon the energy to shout back at her. If he could have, what would he have said? He couldn't tell her about his Mission, especially not in the middle of the Great Hall in front of half the school.
In the end, when his girlfriend of nearly five years stood to walk away, her last words "Don't even think about coming crawling back to me after this!", the only thing Draco really felt was relief.