The back story for the formation of a Spring covenant is a great question, and one that I considered before starting up my Troupe here in Western Michigan.
The backplot I came up with is that our covenant is sponsored by shadowy Arch-Magi. One Arch-Mage in particular, Quintus, wrote an official missive to a nervous young Redcap, Diomedus, telling him to travel to different covenants and recruit specific young Magi who had recently passed their Gauntlet. (Diomedus is my character; I play him intermittently when my Beta runs games, or at council meetings to introduce story ideas.)
The young magi find that they were well-taken-care-of in the laboratory, library, and vis departments. Why, their new covenant was conveniently located in a Magic Aura, and a full map was provided. What generous Arch-Mages!
But what they don't quite realize is that sometime soon, the Arch-Mages expect the magi of the new covenant to repay their kindness. The magi will have a debt of service to pay. When the critical time comes, the Arch-Mages will come calling to settle up...
Be aware that ~some~ mention of "repayment" or "debts owed" should be made formally somewhere - otherwise, the new covenant can simply refuse, and the archmagi are, legally, in no better position than they would be with any Spring covenant - that of bullying and threatening to get what they want. Without some agreement, magi or covenants that give gifts have no legal grounds for complaint if there is no reciprocation, as far as a Tribunal is concerned.
I'd want them to feel some latitude in refusing, and even beyond that, in independently investigating why exactly the Arch-Mages are asking them to do certain things, which itself leads to a fuller Story. They might not perceive this at the outset, but I want to drop hints throughout the story that encourage them to follow their own free will and act in the best interests of their new Covenant.
My original design thoughts on this were more player-centered rather than character-driven. I have two veteran players but three new players, and the veterans understand that they anchor the Troupe in terms of advice and gentle suggestions, but that the three new players are to grow in their comfort level and understanding not only of roleplaying and rules conventions, but also in their ownership of the shared Story. And I have seen this begin to take shape, which is rewarding.
Perhaps not providing a hardcopy of the agreements (which might be colorful, but a bit time consuming, for all concerned), and leaving it as an abstract concept. At the level you're speaking, perhaps some ever-popular "cryptic phrasing" is included, that can easily get overlooked. That way, only characters who 1) state that they are going to read it closely, and 2) have a good grasp of both Latin* and Hermetic Law have a chance to spot it.
(* Need Latin 5, or 4 + an appropriate specialization. "Latin 4" is "functional", on "non-technical" topics, hardly adequate to unravel an intentionally obfuscated contract, thus why many magi take a Latin specialty relating to Hermetic or Magical matters - or study it up to "5", or more.)
Along with this, provide a regular shipment of some badly needed resources from these Covenants- vis, moneys, food(!) - which stop (as per the obscure but legal wording in the Charter) if the players don't do what is desired, what is stated (if also hidden) in the Charter.
