>> This tragedy is very mysterious. How can they
>> fall from the bridge? Do we ever know the truth?

I have a theory... I have no evidence for this but if you rule out some sort of deliberate act, I don't know what else is left.

Here's what I suspect: Brent tried to take a "cool" photo shot by climbing into a dangerous position on the railing, and slipped. He caught himself but was in a precarious position. Drew followed him trying to save Brent and they both fell.

It's possible. I can't imagine Drew not trying to save Brent in this circumstance. It's what I'd do...