well, you did the right thing - the key is generally to get your feet out of your bindings and then just wallow your way out.
everyone keep in mind that in an avalanche burial you typically have about 15 minutes of air, and that snow is compacted like concrete. in a treewell, even headfirst - you have time. stay calm and get out of your bindings.
if you said the words "last run" then that is almost surely why this happened. don't even say it to yourself in your mind - tell yourself "well self, lets take a run and we'll see how we feel at the bottom".
NEVER say last run.