Explanation on why some Legendary variants are 80-s themed and why they're unlocked in Infinity (contains spoilers)

In one of the Plants' final quests, Dave-Bot 3000.1 tells you that in the mid 1980s three of their top agents were partying so hard and were sucked into Infinity.

Now look at the designs for Party Rose, the Legendary Citron and Legendary Corn. They're 80's themed. They can be unlocked by getting shards in Infinity

Therefore, they were the three agents sucked into Infnity. They were captured by the gnomes, obviously.