VIX and therefore UVXY are inversely correlated to SPX movements, but correlation is rarely perfect. The historical average is a correlation of -.80, or 80% of the time they move in opposite directions. This leaves 20% of trading sessions where they move the same direction, moving up or down together. For traders expecting perfect correlation they wonder why prices aren’t moving the way they are ‘supposed’ to. Manipulation! Broken! P.O.S. product! Rigged! These are among the nefarious causes cited as to why the inverse relationship has disappeared for one or more trading sessions.

The causes for positive correlation typically include an upcoming binary event, and recovery from an extreme level.

Binary events can disconnect the relationship as the date of resolution nears.

After a large move during the previous session, many times VIX will adjust during following day (up or down) and track the same direction as SPX