I do not understand why officials always say "definitely not terrorism" before anyone has a chance to investigate.
Because in those (most) cases they are able to confirm relatively quickly that in fact it wasn't terrorism. In other cases they don't rule it out, but say it isn't likely, and so far they have been proven correct in every one of those. However no authorities today have said these 7 explosions are "definitely not terrorism". Not that the facts will ever get in the way of the armchair critics and those who want to scream 'coverup' before a word has been spoken.
On 9/11, the official position just before the 2nd plane hit was that the first collision was caused by faulty navigation gear
The initial impulse is to concoct some plausible story about it not being terrorism, and then see if it can be plausibly maintained. See Flight 800
The subsequent additional explosions in London made the initial official explanation "inoperative"
A government's first response to any situation is to lie.
They can't help it any more than a shark can give up a meat diet.