I was just, in fact, writing about synchronicity in a PM to a friend, when I saw your new thread.
I'm not sure why psychedelics seem to bring it out, but I know that it doesn't only happen on psychedelics. On my +4 with 2C-E, I had a revelation about synchronicity. It happens whenever we are able to see beyond the single frame of reference we're in now so that we experience an event while knowing that it was coming. The universe is laid out perfectly so that everything affects everything else in some way and the whole of it comes together to create what is. As such, IMO, we do have free will at all times, but that free will itself is part of the universe, so that although we can only perceive one frame of it, all time and space and other dimensions are all already in existence, simultaneously. Since our past experiences are what make up our egoes, no matter how many times you went back to a point in the past to make a decision, you would always make the same decision. You'd make the same mental turns, back-and-forths, and the same eventual outcome.
As such, whatever does happen, is what was supposed to happen, so everything is always synchronous, except that most of the time you don't realize it. I've definitely been in mental states where I recognized that everything in my life and around me is always synchronistic.
I have been noticing it more and more and more as time goes by.