What triggers "random" item drops in Dawn of War II?

I've noticed that random item drops (ie those not from bosses) during the campaigns in Dawn of War II / Chaos Rising / Retribution seem to usually happen during heavy fighting.

My theory is that they're triggered by dishing out a particularly high rate of damage (since I've noticed the drops seem to happen more often when I triggering multiple abilities), but it would be nice to have some confirmation of this.

So, is it known exactly what triggers random drops, and if there's anything that can be done to increase the drop rate?

Or is there just a random chance of any given kill dropping an item?

I'm pretty sure they drop more often while you're fighting large battles because there are more enemies in those battles to drop the items. It's a matter of perception that they drop more often, not a boost in the rate.

For example, if you fight two enemies, you have a 2 chances of getting an item. If you're fighting 20 enemies, you have ten times as many chances.