Hi this is a typical weak spot for cracking to occur. However if it was a joint over a door that has cracked the crack would be vertical.
The fact that the crack has gone off at an angle throught the board itself would indicate that some downward pressure exists.
It is quite common in periods of drought for these types of cracks to appear as the ground under foundations drys out and moves.
Fixing the crack just with a filler will not indicate if the problem is worsening or not, due to the fact any crack in plasterboard filled this way without tape will definitly crack again.
There are a few threads below that will describe in detail how to repair this crack.
http://www.woodworkforums.ubeaut.com...ad.php?t=34763
I would repair the crack following those methods and then if it reapprears then you would need to take a serious look at the foundations.
If there were worsening conditions the crack will surely reappear even when fixed correctly. If not you wont have any further problems.
Cheers
Rod Dyson