RQ: Final Soldier Blue Pancake Mystery Bonus

[mjclark]

I've been playing Final Soldier 2 minute Caravan mode recently, and a couple of times out of the fifty or so times I've played a very large blue pancake has appeared when the clock shows 1:10 to go (before the green and blue lifebelt ring appears). The blue pancake seeds enemies down the screen and there's a 4000pt bonus for shooting it. Most times though the pancake doesn't show, so what triggers it?
I havn't taken acid for 20 years so it's probably not that