I suspect a major impact sent the bike tumbling, with impact to the saddle tilting the rigid seatpost within the seat tube.

Now as for future usage, depends on the usual assortment of variables that add up to a definition of a frame's lifespan, mainly loading and # of cycles, but reduced of course because the tubes going into the head lugs are no longer round and straight.

So for moderate use with a relatively light rider, I would expect that it could still endure many years of riding before a fatigue crack starts to appear near one of the bulges.
These things are hella tough to begin with, and it's not like all of this frame's strength has been lost!