DIY Klutz wrote:

QUESTION 1:
Why would this strong torsion spring break all by itself?

QUESTION 2:
If one torsion spring broke - would installing two be better?

QUESTION 3:
Some Internet posts intimate it's a do-it-yourself job;
others provide dire warnings. ...

I simply ask that someone who has done the job themselves, please
advise me on the three questions above.

I haven't done it myself BUT one spring on my door just broke last week
too. My wife watched the guys who fixed it and she thought it really
did take more than one person to do it. They sent 3 guys and it took
them about an hour. They readjusted everything too and it is REALLY
smooth in operation now, much better than before.

As for why they break, well, they're a big spring doing fairly heavy
work in possibly nasty weather. Stuff weakens and breaks under those
conditions.

The spring(s) should be sized to the door.

Do-it-yourself? You might want to search on +injuries +"garage doors".
If I remember right from a past discussion there are tens of thousands
of people injured every year and the serious ones are from when the
torsion stuff fails.