The immediate trigger can be anything. It can be a loud noise or an unexpected event or some small annoyance like accidentally stumbling on a crack in the sidewalk. That immediate trigger is nearly irrelevant, because it's the underlying, dangerously-high stress level that created the situation to begin with.
Thankfully, most parents of autistic children don't have to worry about whether they are dealing with a tantrum or a meltdown. Autistic children can have both; and yes, we can be manipulative children, though we're quite unsubtle about it. The lucky thing is that the response to either tantrum or meltdown is generally the same: Back off, don't interact, and let it wind down on its own. Later, when the child has had some rest (I know I collapse afterward; I'm generally sore and utterly exhausted), you can talk about it and figure out why it happened. If it was a tantrum, then you didn't give the child the attention they were demanding. And if it was a meltdown, you stopped adding to the cognitive load by trying to force them to process it either way.
As adults, the goal is not truly to suppress meltdowns. There's no good in doing that whether you're five or fifty; they're unstoppable once they start, short of a shot of Haldol (and sometimes not even then). Eliminating all stress is impossible, too (and undesirable, since some harmless level of stress is necessary for motivation). The best strategy to learn is to predict them before they happen, to learn to self-monitor and find ways to de-escalate the problem. For me, the best way is to be alone and lie down, preferably on the floor. I have no idea why lying down does it, though I wouldn't be surprised if it were the pressure of the floor against my whole body combined with not needing to process balance and movement as you do when you're sitting or standing (I'm mildly dypraxic). Methods for de-escalating a potential meltdown are varied, and they depend on you and exactly what autistic traits you have. But they're important to learn, and important to teach to your children if you're a parent of someone who has meltdowns.
The fact is, we're autistic and we're always going to be this way, and the world is never going to be the perfect environment we'd want. The best we can do is recognize the problem, define it, and work with the facts as they are, rather than telling ourselves we're immature, impulsive, and undisciplined and hoping we'll somehow "get over it". That's like trying to scold the intertropical convergence zone into not producing hurricanes.