It can't get a grip. Any number of cliches could apply, but essentially slabs move when they rest on a weak layer in the snowpack. The slab may start moving by itself, but often, an unsuspecting victim shakes the snow to start the slide. When snow falls and lands, it's usually made of the fine-grained flakes we'll check out shortly. As soon as the flake lands, it starts changing.
Many flakes undergo metamorhism due to temperature variations.
Imagine you're a snowflake near the ground. The Earth warms you up, and you start to sweat. This vapor migrates to colder snowflakes and recrystallizes. Sam Colbeck, studies snow physics at the U.S. Army Cold Regions Research and Engineering Laboratory in New Hampshire. He explains the process this way.