As a long time 737 captain I would say that a 737 loss from at or near cruise altitude is quite an unusual event. But it does remind me of at least one Indonesian loss of control accident (Adam Air?). Flight level 290 is not that high and the chance of being in instrument flight conditions cannot be excluded. Very few airlines encourage manual flight that high and if the airplane departed controlled flight for some reason then the startled crew might not perform a text book recovery. There is some data suggesting the aircraft made a high speed pull out relatively low with the possibility of over stress and ensuing breakup. But that is just speculation. It is a bit of a short flight for fuel imbalance to be a big issue. But technical failure, plain and simple operator error or of course some exterior factor like turbulence could have caused the initial loss of control. The black boxes will hopefully show up shortly and then we will have something firmer to go on.