We've known these are coming for weeks, but finally the official announcement has rolled in, confirming those leaks we've been listening to all this time. Once again we've encountered a similar situation as the one prefacing the Lowriders announcement. In the weeks leading up to the announcement, leaks and rumors ran rampant, with most of them being utterly fake. It wasn't until the announcement that the few who told the truth were revealed.
The leaks about the upcoming rides and new Adversary Modes began right after the release of the Lowriders: Custom Classics DLC update for GTA Online not too long ago. The game files for the update already contained the in-game models of the new cars and much of the data for the sports-themed competitive modes.
The leaks showed off some of the new assets that will be added with the sports-themed adversary modes and their names also suggested a relation to football. The three new lowriders, on the other hand, were discovered by accident when porting the content of Custom Classics to single player via a mod added not three, but six new cars to the modded Benny's store.

Benny's Original Motorworks was updated once before Custom Classics too. In a small update earlier this year, two sports cars with extensive customization came to the special garage — the Karin Sultan RS and the Bravado Banshee 900R. This first update didn't quite go as planned, however following a quick fix, the new Banshee became the fastest car in the game. It has since been dethroned well and truly however.
Small updates like that began to characterize the DLCs of 2016, with Custom Classics actually counting as a major release, relatively. Even so, Rockstar's one-a-week lowriders DLC model is something entirely new. Spreading smaller nuggets of content out with a more frequent release schedule has the air of building up to a massive punchline about it. Players theorize that Rockstar has something groundbreaking to show off at this year's E3 to back it all up.