Disconnecting an alligator clip sounds "clean" until you see it on a 'scope.
Literally, thousands of "off n onnies" in a fraction of a second. Looks like a burst of rf.
If there's any inductance at all in the circuit, at these currents, the spikes will be higher than the dc level and will be in the wrong direction!
Once you get over a few amps, coiling a lead for strain relief is enough inductance to make trouble.
Then there are small, distributed capacitances...

So, next time, hard wire the leds and use the power switch on the supply to power down and we should be OK.
I've been running 13. 5 Watt leds from my Mastech for months with the supply on a timer.
Keep it set at 12.6V 3.4A.
So I bang the AC on and off with few problems.
(it did occasionally cause my UPS to "freak out" and beeeep at me.)
Weezard, thanks. These details/specifics are what I'm needing. I reordered my supplies and should be back blowing stuff up by the end of the week, although responsibilities in the "real" world may cut into my mad scientist time over the coming weeks. Getting older ain't so bad, but having to act like an actual adult does get tiresome.

I started looking at thermal switches. Most that are referred to that way on the 'Bay open the circuit at a given (rising) temp. The ones that close the circuit seem to be called "thermostats" or fan switches. From your brief description of operation I'm gonna assume the latter is what you're using.

Looking at a pic of your light, looks like you have 3 of them, all mounted in places you'd expect the sink to be the hottest--right near the leds. It also looks like you've got it wired so any one of the switches can complete the circuit by itself. Close? :detective1:

I found some switches that close when temps rise to 50 deg. C. Looks like the right range for me, although certainly I'll hard wire both fans at first.

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DH