So it depends on the spell, but I think you are talking about summon nature's ally. That allows you to give instructions to creatures who can understand, and they will fight to the best of their ability, but as a DM I wouldn't interpret the spell as written to include suicide.

But even then, a good DM doesn't put a tarrasque into play and have it sit there and die. Once it realizes it is getting damaged and can't retaliate, it can burrow from we whence it came, etc.

So I think most of the strategies involve weak roleplay from the DM, munchkin builds, liberties with the rules, or both.

Even then, actually killing the tarrasque requires a wish spell, which is not something that a 9th level druid can do.

How do they manage an average of 679 damage?

First Aerial bombardment rules would probably give the Tarrasque a DC 15 Reflex save for half damage for each. Assuming it was a surprise at first the Tarrasque probably doesn't get this so I'll ignore it.

Second, a Giant owl's likely only weigh like 140lbs by loose calculation, being a little over 4x the height of a snowy owl (so assuming 4 times equivalent weight and then cubed is 64kg which approximately equals 141lbs. It could be a little higher but its not breaking 200lbs) and requiring falling at least 20ft before they even start ranking damage by the srd 3.5 rules for items falling on players (https://www.d20srd.org/srd/environment.htm). Assuming you meant 40ft over the Tarrasque, and allowing for 1d6 damage every 10ft past the point instead of the 20ft that's implied to be required, the owls would deal 2d6 damage each at that height, requiring 20ft of falling to start incurring damage. Even without it that's not 679 damage.

That's pretty much 0 damage too, because 2d6 per owl - subtract the DR 15 of the tarrasque from each instance of damage - is 0 damage. Iirc there was a min 1 damage even for negative strength modifiers but DR superseded that. Even if I'm wrong that's 1 damage per owl max.

Even if you went the 220ft up above the Tarrasque you'd need to hit maximum fall speed under the more polite 1d6/10ft rules, after falling 20ft, you'd end up with 20d6 each, the cap for fall damage. Which after DR is 440 damage.560 damage without DR.

Which actually isn't that high up. I thought the Tarrasque was taller than 50ft, but its still a hell of a timed shot tbh. It assumes the Tarrasque doesn't move for like 6 or 7 rounds, or moves in a straight line into the falling birds.

That doesn't' fix the weakness of a Tarrasque to some form of high impact drop damage, necessarily, just means that I'm suspicious the birds can pull it off.

Summoning animals to kill themselves does not sound like a thing a druid would do

A 9th level druid (any druid) flies 40ft in the air and upcasts one of their summon animals spells to summon 8 giant owls, then makes them fall prone.

I mean, I think the generic DM response to that is going to boil down to "The owls won't behave that way". Its for the same reason DMs blanche at the "Summon a whale 40 ft off the ground" technique (and why rulebooks started adding conditions about where and how things could be summoned, period).

at the end of the day you could also just do this with large rocks

The magic bag full of boulders is a classic munchkin weapon, but it does require some degree of preparation and isn't the same level of mechanically out-of-the-box function as kamikaze owls. Even then, a general DM adjudication to these kinds of techniques would be to limit the damage to an equivalent spell effect. So a 3rd level Shrink Item will let you do (level)d6 damage from a hurled boulder. And you can only practically fling one of these a round.

That allows people to be creative to a degree (dropping a boulder down a mountainside will have different consequences than lobbing a fireball, and everyone can lean into that) without exploiting the mathematical discrepancies of fall damage versus every other kind of damage.