iPads do not spontaneously bend. As illustrated, the iPad chassis has been physically stressed beyond its design capability - and the casing has deformed at its weakest point due to this pressure. The most common explanation is that the iPad while placed on a soft surface has been sat-on - or has been in a backpack with other items and suffered damage.
If you have benefit of an active AppleCare+ plan for your iPad, you may be able to make a claim for damage. If applicable, for the cost of the levied deductible/excess fee, the iPad will be exchanged for a reconditioned iPad of the same model and specification.
Without an AppleCare+ plan the only option is to pay for a repair - this being charged at a flat rate. Again, Apple will typically exchange the damaged iPad for a reconditioned replacement.