Here's my theory on Spider-Man Day:
Gwen Stacy is speaking at the festival (as seen in the theatrical trailer). Also, according to the script review, she is interested in Spider-Man, which I suppose is self-evident.
Spider-Man arrives, does his thing, etc. Then, enter Sandman. Now, however he is introduced into this scene (maybe Spider-Man hears the sirens or maybe the car speeds right by?), it is known that Sandman and Spider-Man get into it.
(Here's where my theory kicks in)
What if Gwen, who is seeking the attention of Spider-Man, follows the pursuit in hopes of catching a glimpse of her hero in action? This distracts Spider-Man, which is obvious to Sandman. Sandman needs to make a get away, so he hurls Gwen up into the air, and gets away as Spider-Man goes to save her. That could make sense. And all of the stuff that Spider-Man is diving through to get to her as she falls is damage that Sandman is creating to further create a distraction.
Here's the picture of her being saved: