Not absolutely certain of this, but my best guess is that, with sufficient fuel on board to leave enough remaining to simulate what the residual fuel level would be on a real re-entry, a single-engined Grasshopper 2 vehicle probably either couldn’t get itself off the ground or couldn’t reach the high altitudes SpaceX is cleared to test at in New Mexico. Three engines preserves balanced thrust and gives enough performance to get the vehicle up high fast enough to make these planned high-altitude tests possible.

Just ’cause no one else is testing like SpaceX doesn’t mean there’s any inherent problems with their design. Historically, the first few launches of a new vehicle are the most likely to break. SpaceX has had basically 2 new vehicles: F9v1.0 and F9v1.1, both of which have performed exceptionally well. There’s no basic issues with any of their design, manufacturing or operations. They would have shown up by now. There has been the odd off-spec item resolved before flight which is also pretty typical of launch vehicles.

For info’, there’s been a delay on the next F9 flight of one day just to be sure that what happened is unrelated to their operational vehicles. SpaceX have a conservative approach to their business, pretty much like others. So far they haven’t screwed up like Soyuz and ESA resently did.

Cheers

This is why I think the explosion here was an unexpected secondary effect of the flight termination, most likely due to due to propellant being dumped too close to an ignition source… something I’ve witnessed first-hand.

the flight termination system automatically terminated the mission

They blew it up for safety reasons on purpose. It did not blow up on it’s own. That’s why NASA requires them to carry explosives on launches for them.