TL/DR - Picard is now a cyborg and accidentally forms the Borg in the far future by trying to prevent death and disease.

For the last century of federation history the Borg Queen has been trying to assimilate Picard. Ever since Q introduced the Borg to humanity she has been after him and has even managed to assimilate him once. This is the main reason why the Borg don’t send a hundred ships and kerb stomp the federation - the Queen needs Picard alive as without him the Borg will never exist.

Throughout his life he has identified more with artificial lifeforms than humans, he almost adopts Data as his son and eventually becomes a perfect blend of the cybernetic and organic (indeed has always been partially cybernetic since his heart was replaced) - a powerful & immortal cybernetic organism with an organic brain. By doing this I posit that he’s accidentally become immortal through this blend of cybernetic and organic parts and that over time as his organic parts wear out the soong family will keep him repaired and continue to replace his organic parts with cybernetic parts.

Picard is also a moral man, his moral compass will eventually mean that he will get tired of his friends getting old/sick and dying when he holds the secret to eternal life. He will then use the soong’s technology to upgrade and repair humans and other members of the federation, eliminating death and disease and saving everyone from the heartbreak he faced when his brother & mother died as well as the pain he saw when Will & Deanna’s son died of a treatable disease.

Over time (possibly centuries) this new cybernetic race will co-operate more and more, building larger and larger structures and new and better forms of propulsion, eventually spreading out into the rest of the galaxy. Eventually a temporal event will thrust a cell of this race back into the past and this cell will rapidly convert new races and new species into their idea of the perfect race where all co-operate and all are equal forming the Borg as we know it.

This even plays out in canon. The Borg Queen is constantly said to be aware of temporal events and changes in the timeline. Every human interaction with the Borg is designed to preserve Jean-Luc PIcard for the future - the Borg chase but not assimilate in Q Who, they send a couple of cubes every now and again to see how Picard is getting on, they ensure First Contact happens (thanks to the enterprise engineers) thereby ensuring the federation exists and Picard becomes an android and the Queen is constantly telling Picard he “thinks in such 3 dimensional terms”. The only thing that doesn’t fit is Best of Both Worlds but this could be explained by the Queen ensuring that Picard has cybernetic devices in him ready for it to accept the Soong upgrades, and to ensure that he knows how to destroy the Borg cube in First Contact so that humanity achieves warp speed and meets the vulcans.

Therefore, it’s not deliberate but Picard accidentally forms the Borg in the future from a desire to protect the galaxy from death and disease and in the process accidentally eventually forms the Borg making it so that the Queen is constantly trying to ensure that he is protected while nudging him along the path to become the Borg's ultimate founder.