If you fail to arm the speedbrakes on the 737 they will come out automatically if you have the thrust levers in idle and select reverse. If you cannot for some reason select reverse, say the thrust levers are not quite closed, then the speedbrakes will not deploy and you will sail down the runway as demontrated here. Anyone know if this is similar on the 757?
The speedbrakes are critical to the stopping distance required. As far as planning is concerned the reversers are nice to have rather than need to have as already pointed out. Of course if you are focussed on why the reversers will not deploy you might overlook the speedbrakes and misinterpret why the end of the runway is fast approaching.
Some esoteric systems failure is always possible and indeed to be hoped for from the pilot's perspective. Hopefully the report should not take too long and then we will all know for sure.