I spent a few hours on MB installing a overflow bottle for the coolant and greasing the sheet winches. A good day except I lost my glasses overboard in 6 metres. It had to happen eventually.
As I was rowing out I saw a new boat. Looks like a harbour boat, nothing onboard to suggest it is a passage maker. Sturdy things these vessels.
What is the consensus on the cause?

Perhaps hoisting the topping lift, rather than the halyard?

Hard to tell from one picture.

It could well have just broken under vang load (ie not up to task). it looks like it probably snapped upwards from the way the alloy has beer canned horizontally at the bottom, maybe it hit a stay as Gravy said, then in its weakened state, snapped.

There's plenty of ways it might have happened though.

must of been cranked pretty tight!

Its a pretty flimsy looking boom to start with, and look where the mainsheet attaches.
the aft half of the boom is unsupported, so it could have snapped trying to get leach tension with the rather powerful looking vang.

I bought a new boom for my Martzcraft 35 2 years ago, it's nearly double the diameter of that one. Sailmalker estimated size to suit sails. So does look very light weight to me.

Proventer or stay the kink in the boom suggests a horazontal load.

Well, a gybe could do it, it does look a bit flimsy.
Pot calling kettle....