I've been quite curious about this after a discussion with Cgamp.
Previously I thought that double leech was caused from crit rate since it can most obviously look like a critical hit or at the very least it seemed like an equivalent to it; however through discussing it with him I learned that certain weapon gems or abbilities which proc upon critical hits do not do so when a double leech occurs thus it's more likely for it to be attached to the hit rate.
Would the admin care to explain for us?