We have a double-glazed sealed unit which has spontaneously cracked on the inside, for no apparent reason. This is the second sealed unit to do this, in almost exactly the same place. It happened the same day it was fitted on both occasions. I don't know if it is relevant, but the glass was wet on the outside straight after the loud crack sound, whereas the neighboring units were perfectly dry.
Long shot, but you haven't upset a neighbour who has access to that window with boiling water? On a very cold day the temperature differential might do it?
...........there goes another theory :o(
well......upvc doesn't usually give this sort of problem.........they do use a metal tool to remove and fit the glazing beads................I can only think they've put a "nick" in the edge of the glass when fitting...........see if you can take the unit out and have a look at the site of the start of the crack.
Surely, if you take the pane out it will validate any guarantee you have with the fitter who repaired it last time. I suggest you get back to whoever fitted it last time. Sorry `Builder`