Why do I cry when a crowd acts in unison? (i.e. standing ovation, tears, common-reaction to an event)...?

You are not alone. Just as "mobs" incite violence, they can also incite other emotions. I have been like this for a long time; haven't really done any research as I was dealing with more debilitating issues (like depression). But I couldn't ever go to Church without crying. The sheer # of people feeling the same thing as you makes the emotion take on a life of it's own. Just an example...I think back to all the people bawling their eyes out in from of Windsor Castle (forgive me if I'm incorrect; not a Brit) for Diana. Probably didn't shed a tear at home. Marketers picked up on this loooooong ago.