The actual chance of detonating an explosive device with a taser type gun, while not zero, is really quite low. I am sure the late Mr. Menezes could have explained this better; but, being the son of an electrician, I can give it a "shot."
The understanding of this fact comes through a grasp of a basic electrical axiom, called "Ohm's Law." Where voltage equals resistance multiplied by current, usually measured in amps; or V=IR for short. Commonly Taser guns use a small quantity of "AA" batteries resting in the grip with a total output of 14 volts. The output is stepped up through the use of induction coils and capacators (similar to an automotive ignition coil) to around 50,000 volts. Since the initial current of 8 "AA" batteries is quite low (they wouldn't even light a common household bulb), and resistance stays constant, the final current, using the above equation, is extremely low.
Explosive detonators, like blasting caps, on the other, hand rely on a relatively low voltage, somewhere around 200 or less, but they need a decent current to push the electrons through the relatively resistive explosive medium. Picture and old time mining switch which is a largish box containing a dynamo, actuated by a "T" handle; it actually takes some effort to depress the handle and energize the circult.
While terrorists are likely to use solid state circuitry and sensetive contact switches to create the ignition circuit, they still would incorporate a decent power supply and the circuit would likely use more that an extremely small fraction of an amp. The possibility that the would employ stepping coils to convert the taser charge to useable current is possible but somewhat absurd. 14 volts would certainly do the trick, but it would have to be shunted differently. The included photo shows how little voltage is required to detonate an explosive device.
The fact that the suspects backpack was thrown out the window indicates the officers believed there was a distinct possibility it contained explosives and it is probable that they incapacatated him with the taser before they took the pack.