Pretty much! Basically, an avalanche happens when the force that the underlying snow layer or terrain puts on the top layer of snow (in the form of friction), is overcome by the force gravity and the weight of the snow (that is pressing down) puts on it. When the hills surface cannot hold the snow anymore, it slides down.
In this case, grass that had been laid down the hill, and soaked through with lots of water from melting snow, was not enough to hold up the huge layer of dense, heavy, spring/man made snow, so it slid down the hill.
Others cases can be different, sometimes snow layers can slide on top of each other. Things such as spring pollen, atmospheric dust, ice crust, different densities, different snow crystal structures, temperature changes, and a billion other things can affect avalanche risks. Generally, the only avalanches triggered by skier or boarders, and not melting snow or a really steep slope (or any of the factors listed before), are ones where a layer or layers of light/heavy snow slide off of a heavy/light base layer. (Note: The densities have to contrast each other, because light snow tends to cohere to the light snow, and vice versa, leaving the light and heavy snow separated).
But in the end, yes, it is basically a big layer of snow sliding off of a slippery base.
