Why did Bill get mad and rip the flower off of Maries dress

It isn't explained of course in the film, but I believe the answer is that they were the same kind of flower (sweet peas, I think) that Lily was planting in her garden. The flower name is mentioned when Marie takes the flowers from the other girl. I think seeing the flowers reminded Bill of who he really loved. The sweet peas make another appearance at the end when Lily tells Bill that she's planning to plant them in the garden - this confirms that the flowers have a special meaning between Lily and Bill