Maybe the high-lift devices malfunctioned?

It seems to climb OK for a little while then suddenly start sinking. There's no sign of an obvious engine problem. Not sure whether we'd be able to see any sign of a bird strike from this far away.

For multi engine planes it's pretty rare, most likely a fuel system failure, or less likely pilot throttling error. My money would be on something with the fuel system.

"Did I fill the water in the right hole on that plane?" -- Guy at the airport driving the freshwater tanker.

I was just thinking about this, perhaps when the aircraft rotated, water or other contaminants got drawn into the fuel system?

Or shifting cargo damaged the fuel lines?

It's always hard to judge AoA unless you're looking side-on with a horizon for reference, but excessive nose-up attitude caused by cargo incorrectly loaded or not secured properly so it shifted aft during rotation could have caused an aerodynamic stall.

Wake turbulence?

Maybe, maybe not.

What’s way weirder is that he’s got zero flaps and gear is still down, which is the exact opposite of what you want to do when climbing out. Maybe pilot error (control inversion)?