I find it a bit weird too though it doesn't bother me massively.
I have been using the internet for almost 20 years, and I used to go on a message forum some 15-16 years ago, and they used to say DH, DP, DD, DGS1, and DGS2, and so on, so it goes back to the early days of message forums from the turn of the century.
I think for many it is easier than saying husband, partner, daughter, grandson 1, and grandson 2 etc. But as has been said, it is odd when someone says my DP has been abusing me, because it's like 'why call him DEAR then?!' 
But yeah, when people say Ddog and Dniece it's ridiculous. It actually looks so daft.
Some abbreviations are confusing though. DSIL - sister-in-law? son-in-law? DF - father or fiancé? BF - boyfriend or best friend?
This ^ I think it would be better if people just started writing the proper word(s) because as some people have said on here, it can be confusing, with DF meaning dear father or dear fiance, and BF meaning best friend or boyfriend, and so on. And when I see GP, is it grandparents or general practitioner? 
But I don't see people dropping the acronym game anytime soon, as so many people have got into the habit over so many years.