At today’s inspection in a crawlspace, I saw sagging flex ducts, which is common. However, one appeared odd due to the sharp kink in the duct. I pushed up on the duct and it was full of water!
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Randy, the first thing that popped into my mind was floor mounted return/supply registers in a “flood” prone area such as bath, laundry room etc. My two main level baths, laundry and kitchen have floor mounted registers. I have had my washer in the laundry room overflow twice, but I caught it both times before it reached the register. Just a possibility. 
If they have a damper on that run choked down or closed, it can build up a lot of condensation.
Good catch Randy!
do they have a large dog ???
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I came across this 10+ times in Oregon over the years. Honestly, I figure I missed it a few times since it was very common for ducts to be sitting on the floor of a tight crawl space and I definitely don’t go push on every duct. I suppose if it were totally full I’d notice the lack of heat.
I followed up a couple times agents to ask what was ever found to be the cause and it was always carpet cleaners mistaking the duct openings for floor drains… oops! 
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I had a house some years back that used a flexible metal duct for a gas dryer vent. As the duct approached the exterior wall it dipped like a plumbing trap. Needless to say it was full of water.
Slab on grade homes with in slab ducts usually have water and debris in the ducts.
I inspected a house slab on grade that when the AC blower kicked on, gurgling noises could be heard from the registers, this was due to high water tables in the area. The seller had 3 sump pit systems around the exterior of the house, they didn’t solve the problem.
This time of year with the dryer air, they must have really had the humidifier cranking to cause puddling in the crawlspace ducktwork, I don’t know what else could cause it, unless the carpet cleaner guy was using the floor registers as floor drains like in the post from @mfellman above. 
Evidently a few too many cleaning chemicals being inhaled, lol.
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Many years ago, I inspected the cat lady’s house. Easily 50 or more cats. In the unfinished basement, I noticed a flex duct was moving and closer examination found several cats lounging in it.
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A heated hammock. Sounds like an ideal place to be in the winter.
Randy,
I am not sure where you live, But we have them randomly in our area, (SW Indiana). It is caused - sometimes (if nothing spilled into duct), by condensation off the metal boot to floor. The boot is normally uninsulated, next to an exterior wall and often next to a foundation vent. A few factors on this also contribute…extra cool setting of AC, a non insulated crawl.
Usually this is remedied by insulating the boot and ensuring AC settings not set too cold, proper Freon and straightening out duct.
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