Hi @DJH2389
I too have had my phone since Launch and have an official Samsung Black Leather Back Case on it along with a Whitestone Dome Glass Screen Protector.
May I ask.....
What case do you have fitted on your Samsung phone ?
We're you using fast / superfast charging ? I ask as this would naturally make the phone feel warmer to the touch.....
....... however this shouldn't lead to the back glass plate cracking !
Accidental damage isn't covered under the Samsung Limited 2 Year Manufacturers Warranty unfortunately but as this isn't connected to accidental damage you could ask a Samsung Experience Store or Samsung Service Centre to take a look to see if there was a weak point in the manufacturer of the back plate.
This will be considered at the discretion of the Service Centre or by contacting Samsung Support direct to send the phone to them.
Please contact / visit them at your earliest opportunity.
Ready yourself for a possible repair cost if they deem this as accidental damage. I'm in no way saying it is my friend but they may attribute this as so.
Also check any Household insurance or some Bank Accounts provide cover as a perk of the account to see if your covered this way.
I've seen threads in the past where either the back plate or indeed the front display has cracked / shattered and it's ultimately at the discretion of Samsung on how they determine how they offer support.
I'm a customer like you so I too would be unimpressed if my phone suffered the same catastrophic damage as yours as I do also look after my phone's.
Would you let us know how you get on with this ?
I wish you all the best. 
