it's extremely unlikely that it's her fault
we can't say who or what did it but it's very likely her dad signed her up for all of this
you know, without really ever asking probably so it's like CONGRATULATIONS
U R THE LITTLE WARRIOR I RAISED BB
NOW GO BE A TITAN MURDERBEAST

also no one knows yet why titans exist so no one knows who caused it or WHY THERE IS ONE THING THAT MAKES IT HAPPEN SO MANY MYSTERIES
AND WE STILL NEED TO LEARN WHAT IS IN THE BASEMENT
which makes zero sense when you're canon unfamiliar but trust me it was once the goal
getting to a basement