Her friends are busy with their lives. Her sister is busy with her work. She is busy with trying to cope with the feeling of being alone.
Sweetie Belle is bored with the solitude for the weekend, but as it happens she has a magic book lended to her by Twilight to practice her magic. While browsing it for amusement she discovers something mighty interesting. Summoning spells! So of course she tries it out.
She didn't, however expect to summon a helmet. Or rather a helmet being worn by a talking severed head.