How does Riot choose what champion gets an ultimate skin?

So a friend and I were discussing (with the upcoming skin) the state of Ultimate skins and Legendary skins. To shorten a lot of this conversation, we ended up talking about how Riot chooses who gets what skins, and how we believe this works.

My friend says it seems that Riot gives Ultimates to the champions who need a lot of love-- with gameplay or visual updates but aren't planned in the near future (say, a year) or so ( I could see this with but not who got a rework eight months before her DJ skin came out for season 5; I did the research for this). I say that it's counter intuitive, and I remember reading somewhere during a Q&A about championship team skins that Riot doesn't give champions skins with impending visual reworks or those who might already have other planned skins in the near future, with the exception of those particular championship team skins (I.E. even though say Malzahar could get a team skin, it's still possible he could receive another skin if it just so happens to happen at the same time (he doesnt actually have one, just an example))

TLDR;

What kinds of decisions lead up to a choice in skins in general? Is the decision of ultimate skins based on something else, or is it just whenever a passing fancy happens to coincide with a good time to release?

Bonus points for answering: Do you only do one ultimate each season / no more than one a season? Or is it possible, if it happens to be fate, that another ultimate could happen within a year of each other?