I agree that an explosive device somewhere on the planes (perhaps planted somewhere on the planes, in the luggage, or on someone's person) is a likely explaination. So is a suicidal hijacker. The explosive could've been planted by an airport employee or smuggled.

Sabotage of the engines or hydraulic systems is harder to cause a catestrophic failure at a later date. First, it requires access to the engines and/or hydraulics, such as mechanics on the ground. That means you have to have an insider working at the airport. It also means your activities could be discovered since mechanics seldom if ever work on an airliner alone.

Like I said before, whatever happened appears to have happened quickly since the crews didn't get off much in the way of distress calls. For a hyraulic system failure to cause a rapid catestrophic failure, it would've had to be something like causing a full deflection control movement or activation of a thrust reverser. Not impossible but seems unlikely, although an Air Lauda airliner crashed some years ago due to the accidental deployment of a thrust reverser in flight (terrorist groups tried to claim responsibility but it was a purely mechanical failure).