Applejack, AKA AJ AKA Silly Pony, is farmer who faces down many, MANY problems. These problems include, but are certainly not limited to: financial worries, family support, crop yield, and crabs the size of apples who devour entire orchards in the blink of an eye. And possess poisonous shells. So, just like any other pest a farmer can reasonably expect to attract.
The Apple-Crab (not to be confused with the Crab-Apple, which is more annoying than fatally poisonous and highly aggressive) is a pest that recently has been plaguing the farming community of Equestria ever since some thoughtless berk tried to introduce some comically amusing animals into the wilds of Equestria, apparently for a bet. Sweet Apple Acres has so far been lucky to not be visited by such a creature, but unfortunately their luck seems to have run out, as almost all of the entire East Orchard is being chowed down by this rather geographically inaccurate crustacean.
And it couldn't have happened at a worse possible time either, because it just so happens that you were on an errand to Ponyville that day, fixing somebody's roof via sudoku and advanced trigonometry (it's a heck of a roof, don't ask further), and you are obliged, as a sort of acquaintance and official Royal Dispute Settler, to track down the elusive and dangerous creature, while also not being expected to charge full price for your services to Applejack, curse the friend discount.
Oh well. Better get that roof fixed quickly then. Applejack's livelihood is at stake, and Celestia be darned if her cider and apple pie isn't worth preserving...