From my experience with them, the mines do direct hull damage over time. If you hit a target with multiple mines, what's supposed to happen is that it extends the stun/damage over time length.
What's *actually* happening if that it both extends and *multiplies* the stun and damage over time length and strength.
EDIT: So for example, let's say you take 100 kinetic damage/second when hit by a mine for 5 seconds. When you get hit by a second mind during that duration, it's supposed to add an additional 5 seconds onto that length, but do nothing else.
Instead it's both adding an additional 5 seconds onto the length, but also compounding the strength. So rather than take 100 kinetic damage/second for 5 seconds, you're taking 200 d/s for 10 seconds. If you dispersal pattern for 7 mines, you're taking 700/s for 35 seconds.