The fade is activating. 'Start fade' makes it rewind to the beginning of the fade and play from there (when it's opaque). The condition 'Variable less than 0' isn't just true once. It's true all the time. The events are checked once every tick (generally 60-100 times per second), so you're repeating the 'start fade' action over and over again.
Add the 'Trigger once' system condition below the condition, and it makes it true only once, even when it repeats. It makes it act like a trigger, so it runs just once the first time it's true, instead of every tick.