A creature's movement provokes attacks of opportunity normally during mountain avalanche
To this reader it seems that a martial adept that initiates the 5th-level Stone Dragon maneuver mountain avalanche [strike] (Tome of Battle 84) provokes attacks of opportunity normally due to her movement.
However, keep in mind that a typical creature's movement—no matter where to, what kind, or how much—only provokes an attack of opportunity from each typical foe once per round. So, for example, while the martial adept may provoke from enemies for leaving a square those enemies threaten to enter an enemy's square, the martial adept won't typically provoke a second attack of opportunity for her movement from those same enemies for exiting that enemy's square.
The Player's Handbook on Attacks of Opportunity, in part, says
This feat [i.e. Combat Reflexes] does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you—such as by moving out of a threatened square and then casting a spell in a threatened square—you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn’t count as more than one opportunity for that opponent. (137–8)
For example, a human martial adept faces off against a kobold that possesses the feat Combat Reflexes and that's armed with a longspear and wearing spiked armor. She takes a full-round action to initiate the mountain avalanche maneuver, and, as part of that maneuver, begins moving. She enters the kobold's threatened area, not provoking an attack of opportunity.
Then the martial adept continues her movement but within the kobold's threatened area so that she's adjacent to the kobold, provoking an attack of opportunity from the space she left due to the kobold's longspear's reach. The kobold makes that attack of opportunity and misses.
The martial adept's movement thus far resolved, she continues moving, traveling into the kobold's square. (This is not normally allowed, but the maneuver creates an exception.) Were it not for the Player's Handbook clarification, this, too, would seem to provoke an attack of opportunity due to the kobold's armor spikes, but the martial adept's movement is here merely providing the kobold same opportunity it already had—an attack of opportunity due to the adept leaving a threatened square. The kobold can't make a second attack of opportunity against the adept. The martial adept stomps on the kobold with impunity, dealing it the mountain avalanche maneuver's damage.
Then the martial adept continues her movement, exiting the kobold's square beyond the kobold. Since creatures that wield nonreach weapons—like armor spikes—threaten their own spaces, the martial adept would seem to provoke from the kobold a third attack of opportunity, but this is still just movement on the martial adept's part, so the kobold doesn't get another attack of opportunity against her.
To be clear, a creature can't normally move through a foe's square, but doing so is specifically allowed by the maneuver mountain avalanche, and its specific rules cover what happens when the adept does. Unless the martial adept only occupies a some of a big creature's space as the maneuver describes, entering the creature's space due to the maneuver doesn't itself offer the foe a new and unique attack of opportunity—only the movement provokes, and, as described above, it provokes normally, typically only once per foe per round.
"Okay, but this still sounds like a pretty terrible maneuver. When would it be useful?"
In a campaign I'm running right now, Shady Stu is a level 14 mishmash of pugilist, scout, swordsage, warblade, blade dancer, shadowdancer, and totemic demon slayer with a speed of about 150 ft. and a Tumble skill modifier in the +40s. Stu could use this maneuver to murder whole villages. (He wouldn't, though, unless they were evil villages; he's a good guy.) For some players, I imagine that alone makes the maneuver appealing.
However, I'm guessing there's no Stu (or a character like him) in your campaign, and if there's not, generally, you're right: this maneuver isn't very useful. Its damage is, at the level it comes online, very nearly insignificant (even if it is almost guaranteed), and the risks that come with it can be… excessive.
Nonetheless, many martial characters that specialize in melee combat lack completely any way of affecting multiple creatures with their attacks. They've to hope to be surrounded by a horde of enemies if they're to take out more than one per turn. Further, there are DMs who throw hordes of weak creatures at their PCs, maybe hoping those weak creatures will soften up the PCs with the occasional natural 20. Those weak hordes still need to be dispatched, and the wizard can only cast each day so many fireballs. In such campaigns—likely the same campaigns wherein PCs consider the feat Great Cleave a valid lifestyle choice—, the mountain avalanche maneuver may be an attractive, if overpriced, choice.