The thing that causes an avalanche, the loud noise or whatever it was.
You could try and blame the snowflake for being there, but even if that was a valid criticism it would only give them limited responsibility for the avalanche happening. Blaming the snowflake is like blaming tinder for the fire, when without the spark no fire would have happened.
Yes, something needs to trigger it.
Thinking a bit more though, I was only thinking of a snowflake in the avalanche, rather than a snowflake falling on the top causing everything to fall down - like messing up the last card in a house of cards. If that’s what they meant then it makes a little more sense, but still doesn’t really hold true. 90% of all avalanche disasters are triggered by humans.
An avalanche requires that certain types of snowflake form a “weak layer” in the snow. Some snowflakes are kind of smooth on the sides, these don’t have the jagged edges that hook onto other snowflakes. When a force is applied, this weak layer breaks and the snow on top of the layer slides down the slope. A single snowflake will not apply enough force to break the weak layer - the amount of force it applies would be negligible even compared to things like the wind. Something else will trigger the avalanche before a snowflake ever could.
The snowflake provides the conditions for an avalanche, but doesn’t apply the force that triggers it.
