Here it is:
If balloon lady was part of it she would be leading the parade. She passes the striped sleeved lady, stands next to her and lets the two bombers go by. The very second Djokhar passes by, striped sleeve lady begins to fiddle with something in her hand, probably a phone.
Looks intriguing, but could simply be a coincidence. Lots of people walking around, standing around, etc. The timing of balloon lady and striped sleeve lady however, seems odd.
Oh wait....I found it and did a quick screen cap. Looks like the balloons go into the air as a result of the bombing. Still interesting.
Of course if stripped sleeve lady was the trigger she didn't need the balloons as a signal since she was right there.
Use the balloons to signal to the others where the bomb is. Release the balloons seconds before detonation.