Her airs were allegedly foul-smelling. You may have also read that he described her as a "Flanders mare" (i.e. She looks like a horse), and that he found her so repulsive that he was physically incapable of consummating their union.

It's also important to note what happened when they met. She was brought to a castle in Rochester. Without introduction, Henry came up to her and kissed her. Henry was a big believer in love at first sight (when it came to him, at least) and so he assumed she would just know this was her betrothed and they were meant to be. All Anne saw was a much older man who was not in the prime of his own beauty (and who, allegedly, you could smell before you saw him due to a festering leg wound) and so she was horrified and rejected him. As soon as she learned this was, indeed, her husband-to-be, she was apologetic and contrite, but it seems it wad too late to amend the first impression she made, which was to injure his pride. After all, he had thought her beautiful when he saw the portrait made of her by Holbein, who was widely respected for his accuracy in portraiture. All that seems to have changed was his encounter with her.