How?

So, my mom got her identity stolen apparently, and they have taken around $1400, which we should be getting back, but it still makes me wonder how it happened. My mom assumes it was because she uses her card to buy everything, and usually on the internet. But I don't how that works, because they made a paypal account linked to her bank account, which they would need access to her bank account and the numbers on the bottom of her checks to verify it. So who know, but hopefully the criminal is caught soon.