It was fashionable long before that. IIRC Mint had an embarrassing start being advertised as particularly secure (great for online banking!!) despite not really offering anything good in that department.
Mint was just weird back then. Ubuntu was so young and had so much things going on, and then came Mint sipping a bit of that energy away. Some users would end up in the Ubuntu support forum I was active at the time and mention they use Mint, and we were like "why would you do that?" In my bubble it felt like a diverging amateur approach (it felt, no idea about whether that was the case) right at the wrong moment with misleading advertisement. That was still very early days for Ubuntu, when users still had to be educated not to break their system with strange and insecure tweak bash scripts downloaded from other forums. And I'd argue it was also before one could understand why you'd want to fork Ubuntu, before there were controversial decisions - especially not with "we make the desktop easier!", Ubuntu was already doing that.
Later on Mint developed its own profile, this was way before Cinnamon.
The "security" thing is deceptive, although I don't see that any worse than what's currently on ElementaryOS's frontpage[0]:
>Safe & Secure
>We’re built on GNU/Linux, one of the most secure systems in the world. It’s the same software powering the U.S Department of Defense, the Bank of China, and more.
Later on Mint developed its own profile, this was way before Cinnamon.