Cracked iPhone Screen. just appeared not been dropped had case on and strongly believe it not my fault

About a year ago my iPhone got swapped out due to cosmetic damage.

The guy said they do it once throughout the iPhone's lifetime.

I had originally took it in because of software issues.

This was at Covent Garden in London.

I've had issues hearing people for a long time on my iPhone and it echoes.

This morning I noticed a crack on the top left hand side of my phone.

It starts at the ear piece speaker, just narrowly misses the front sensor because the crack goes underneath it, then finishes on the left hand side.

I'M CONFUSED BECAUSE i.VE HAD A CASE ON IT AND IT HASN'T BEEN DROPPED, IT JUST LITERALLY APPEARED. Could the heat do it?

I've got a call scheduled for tomorrow afternoon and I don't know what to say, I strongly believe it is not my fault.

I know Apple have amazing Customer service skills, but what are the chances of them swapping it over?

Any Help or advice would be muchly appreciated.