You see, if there's one thing that never really made much sense to me about the Storm King, then it is how exactly he caused the big storm that sunk that ship in 'Spring Breakdown'. I mean, he didn't have any magic of his own, and Twilight's recounting of the events doesn't seem all that accurate to what actually happened in the movie. I mean, she said something about sealing his magic into the staff of sacanas, not...you know, him shattering into pieces.

However, I think I've figured it out!

The key is this moment.

At the climax of the movie, the Storm King uses the staff to create a massive magical storm that nearly tore Canterlot to shreds...and even after knocking the staff out of his claws, the storm keeps raging. However, once Twilight gets a hold of the staff, she manages to use it to absorb all of that storm magic back into the staff and make the weather normal again.

But...maybe some of the magic in that storm managed to find the portal to the human world in the Everfree Forest and escape through it before Twilight could get the staff. While it was probably a very small amount of it, once in the human world, the magical storm began to gradually build up over time.

So in conclusion, I think the magic that Twilight said she sealed in the staff, as well as what attacked the ship in Spring Breakdown, was the storm itself, not the Storm King's own magic...which he didn't actually have.

That makes sense.

Though I don't feel motivated to go by Spring Breakdown as canon for multiple reasons including making Rainbow Dash the villain before the true threat emerged at the end. The only bit of redeeming value for me - aside from the Equestria Girls turning into ponies and having tails for the first time ever (except in Sunset's case, and Sci-Twi if you think turning into Midnight Sparkle counts) - is Twilight mentioning the Storm King having been destroyed.