What caused this seat clamp / frame failure? Here's my "long distance" ride, which is an old steel MTB without suspension.

I picked it up by the saddle to move it and felt some play, I thought the saddle rails might have been loose, but as per picture, there's a crack 90% of the way around the frame. 

I have bent it open a bit more with medium finger pressure.

The crack shows evidence of rust, so its been a slow break rather than a sudden one.



I'm 100 kilos and have long legs, with a standover height of 91 cm.  The frame is too small for me at 20" / 51 cm.

The seat post was reinforced internally with extra pipe (I have bent seat posts before).
The post extended 10cm into the frame (i.e., well past the crack) and then 29cm up to the seat.

Strava says I've done 2,949 km on this bike in the last 15 months.

So my main question: is this an old-age fatigue failure of the frame, or did I break it by being too tall, or too heavy?

Second – is the frame repairable or is it now scrap?
It looks like plain old metal fatigue. Most probably there was a hairline crack, then water got into it and rusted it out. 

Note that when you extend the seat height, you increase the lever arm of the seat post and put a considerable amount of stress on the frame precisely at that location. It doesn't help that the design of that seat clamp has no reinforcing material and is high-up, further increasing the leverage on it. Poor design.

The good news is that it's a steel frame. You could take it to an auto body welder and they could easily fix it for less than an hour of labor. Since they can't weld from both sides (the seat tube wouldn't go in), it'd still be weak and susceptible to damage. Or you could try rebrazing it yourself.