The $50 is simply a temporary addition to your Westjet reward dollars account. So basically you can only use them towards flight fare and I believe WJ Vacations packages.
Definitely random. My wife's account got the email 2 weeks ago but mine didn't. I have had the reverse as well. One time a couple of years ago by fluke it lined up that I received the temporary bonus at the same time that they had a decent price available for somewhere we were planning to go.