Mind worm spam at endgame

I've been enjoying some SMAC recently and I've frequently been confronted by massive swarms of mind worms towards the end of the game. I presume that this is related to the "growth dream" you read about in many of the Book of Planet interludes. I've encountered swarms of up to 30 mind worm units which can devastate even a 24-pop base. This forced me to pull forces from the front line with other human factions. Luckily I won with Transcendence before they could deal critical damage.

What triggers the arrival and/or size of these giant swarms? Is it purely based on M.Y.? Specific technologies or secret projects? Ecological damage? Next time I am thinking that I wish to have a lot more garrison units available to protect my bases and I would like to have a better understanding of the timing so that I'm not desperately trying to produce garrison units too late.