Queen Chrysalis, the sole ruler of Changelings, knows that if things stay the same, her hive will starve to death.
The hunting grounds, used for centuries by her hive, the hive of her mother, the hive of her grandmother, and for the entirety of the Changeling Hive's existence, have run dry. Her hunters have to search further and further... but it's not enough.
She has to expand, and the closest nation is that of Equestria. Her hive must come first.