'Sturm' is the German term for 'assault' (to use another WWII era term, the Sturmgeschütz, or StuG, vehicles were a line of assault-guns, and Sturmgeschütz literally translates to 'assault-gun'), so it's not the 'storm' you might be thinking of. It's 'storm' in the sense of 'storming a building' rather than vicious weather. As for why they were fond of using it, I have plenty of theories, but I imagine none of those would qualify as well-researched answers and thus aren't appropriate for the subreddit.