About a year ago during the bull run there was a very clear correlation between Alts and BTC, one would go up, the other would go dow. Why is this and what has changed?
Actually,,, there were 3 correlative patterns, not just the one you mentioned. The first is as you describe, the second is described by the first response to you which also points out the inverse correlation sometimes witnessed. And finally, there are also periods where no movement is happening with alts while BTC goes up or down a lot.
1, BTC goes up/down and alts follow. This is the general correlation, because crypto = BTC. If BTC does well, then in general alts go up, sometimes more so than BTC and if BTC has bad news, confidence is worse in alts.
2, BTC goes up/down and alts do the opposite. This is less usual, but happens because when people buy alts, it is generally in favor to BTC. Indirectly, choosing alts over BTC pushes their demand and price.
3, BTC moves and alts do not. This is the least usual and is just when BTC is being crazy or manipulated