What's the catch on Siren's "Sub-Sequence" skill?

It reads: "When an enemy dies under the effects of Phaselock, there is a chance for your Phaselock to seek out and affect another target (20% chance per level)." Since it has 5 levels, that means it can potentially have a 100% chance of seeking another target, but that sounds a little too powerful to not be the last skill in the tree, so I figure there must be some small detail that would make the skill not so viable.

The thing is, if it does exactly as it says, I'd only need to phaselock once per fight if me and my teammates kill the target fast enough, and every target we'd shoot would be stunlock until there is nobody left.

Since I think that's way too powerful compared to the trees/classes' skills of the same tier, there must be some catch. Is it that it will only seek one other victim per cast before disappearing? Is it that the duration of the lock isn't reset everytime it seeks another target? Is it something else?