Hot cables

Here's the scenario:
100' of 10/5 SOOW ending in an L21-30C. L21-30P--12" of 10/3 SOOW--*--L6-30C adaptor. L6-30P--10' of 12/3 SJOOW--2500W HMI followspot.

After a few hours of operation, I felt all along the length of the cable that I could reach. Near the female L6-30, at the point of the asterisk above, the cable was hot, not warm, hot; too hot to hold comfortably. Swapped out the adapter and things were better. The new adapter got warm, but not hot. Neither the 10/5 nor the 12/3 seemed above ambient with either the old or new adapters.

Why did the 12" of 10/3 get hot before the 10/5 or 12/3?
How is 12/3 SJOOW permitted on a luminaire that draws "up to 30A (200-240V)" according to the manufacturer?

You almost got me Derek. I had a whole thing typed out about SJ vs SO cable and then noticed where this thread was posted.

Increased resistance in the 10/3 cable, probably caused by broken strands. You said the cable got excessively hot, not the connector so it's not likely to be loose screws or contacts in the connector.

Under what circumstances will a 2.5kw fixture draw 30A at 240V? The NEC permits a 12AWG flexible cord with 2 current carrying conductors to carry 25A which is 5.2kva at 208V. (I would not expect 240VAC to be present at a L21-30)

I wouldn't discount loose contact screws so easily. The copper conductors in the cable are better heatsinks than the little stubs of brass of the contacts in the connector, and the connector body has quite a bit of surface are to allow it to cool.