The thrust reverser is driven by an air motor powered with pneudraulic (air) pressure drawn from a bleed air duct off the compressor. Connection between the air motor and the reverser cowls is accomplished with the use of flexshafts (flexible drive shafts).
The next morning about an hour after the jet engines were started, the hydraulic fluid now under pressure would drip from the cut line until none was left in the line and the thrust reverser would simply slowly drift into the full on condition while in flight and a catastrophic crash would occur only seconds later.
Hydraulic fluid under pressure wouldn't drip from a cut or severed line. It would fly out of there, rather like a severed artery. Any falling onto the hot section would cause a massive amount of smoke to come from the engine, enough that the ground start crew would stop the startup procedure. If it leaked out overnight, then it would have leaked through the cowling as well, and left a puddle on the ground.
With regards to the thrust reverser cowl "simply slowly" drifting into the full on condition. It would have to be driven into position by the flexshafts. At full power, depending on the model, the CF6 puts out between 40,000 and 72,000 pounds of thrust. Reverser cowls are driven into place and held there...because that much thrust will just blow them open.
The fan duct lines in the CF6-80C2 create pressure forces such that the thrust reverser should remain stowed (and not drift back) during high power flight.
If the reverser was the cause, we would have heard about it already. An earlier poster noted that the NTSB stated the T/R's had not deployed. [but then some folks don't believe the NTSB or other alphabet agencies!].