I've been naming my files as "some_keyword_8k.jpg',

now, my latest submission got rejected, and the reason is not explained anywhere...

Anyone know what that means?

Yes it means you don't get to feed the free image/video beast with your file 

It was a bug:

Quote
Thank you very much for contacting us; we apologize for this.

It seems that there is a bug in our website. Our IT team is aware of the situation and currently working on the issue - please bear with us.

Sorry for the inconvenience and thank you for your understanding