If you have ended up with a cracked iPhone then this could have been a result of anything. In many cases it won't have been your fault and the phone will have just dropped out of your hands while you were using it. In some cases you can even end up with a cracked iPhone while the phone is still in your pocket and this happens because you end up accidentally walking into something - such as the corner of a table - which then skewers and breaks the screen of the phone.
However while in many cases you will not be responsible for the damage to your phone, in other cases a cracked iPhone is a result of negligence to an extent or to you using the phone somewhat recklessly. For instance if you are using the phone while multitasking - for instance trying to send a text with one hand while doing something else such as cooking or eating is a recipe for damaging the phone, while at the same time if you are craning to take a photo with one hand then this is a short way to get a cracked iPhone too.
However while in many cases you will not be responsible for the damage to your phone, in other cases a cracked iPhone is a result of negligence to an extent or to you using the phone somewhat recklessly. For instance if you are using the phone while multitasking - for instance trying to send a text with one hand while doing something else such as cooking or eating is a recipe for damaging the phone, while at the same time if you are craning to take a photo with one hand then this is a short way to get a cracked iPhone too.