Here is a closer picture

Are you sure those leaves aren't damaged from a freeze? I would cut away all the leaves with a pruner, then inspect the stem for anything obviously bad like rot, although it doesn't look like rot.

Check the undersides of the leaves for mites, scale, or mealies just to be safe, although once again it doesn't look like damage from sucking creatures.

If everything else seems OK after you cleaned away the dead leaves, give your cycad a healthy dose of high nitrogen fertilizer, like lawn fertilizer (make sure it has no weed killer or anything like that).

The nitro, plus the pruning, will be enough to trigger a new flush of leaves within 4-8wks as long as it is warm. If a new healthy set of leaves forms for you, then you'll know there was some sort of physical damage to those leaves like a freeze.

If all else fails, try to find a local cycad nursery or club that can help you. I have no such thing in NY, LOL!

Good luck!

Wow, that definitely does look like it is some type of disease or bug damage. Some of the really good local nursuries around here will allow people to bring in a piece of their damaged plant and/or a pic of it and let their staff figure out what is wrong with it so if you can find a nursery by you that does the same, that would help a lot.
If you can't find anything on the fronds, then the problem could be taking place underground but I am really not 100 percent sure. If you had a freeze this winter, it could be delayed cold damage, but usually the fronds don't look like that from cold.
Good luck, I hope you figure it out! If I had that problem I would cut the damaged fronds and if the fronds keep dying off I would consider cutting off all the fronds. Usually when they have no fronds they will make a new flush of fronds within a few months, but if it is diseased then doing that will only give it more stress.
-Alex

I don't think it is freeze damage as we don't get hard freezes here.

Might also be a nutrient deficiencies like manganese. Also is the water from the sprinkler rather cold? Sagos can be weird about getting cold water on the leaves when there is hot weather.

Nutrient deficiencies should manifest themselves in new growth. I am under the assumption that these yellow leaves were normal when they first formed, is that correct OP?

It is bizarre. Doesn't seem to follow logic. Insect or sucking animal damage is usually slow, not sudden. It looks to me like an event occurred that quickly led to the damage, like a burn, freeze, poison or chemical.

You can seek advice from someone local, or like I suggested, snip off all the leaves and wait to see if it recovers. If new growth is normal, then it was an event. If new leaves are not normal, it was disease, insects or nutrient problem.

Looks like the first one got sat on by an elephant( :

A strong opinion looks like a nutrient deficiency of the mobile group as the older leaves are showing the symptoms first rather than the immature leaves.

You can try amending your soil with something that will get the pH to slightly acidic or slightly alkaline at least since cycas revoluta will prefer this range. You can add mycorrhiza innoculant to benefit the plant in its uptake of nutrients but it's not necessary. Then get a complete fertilizer high in N.

I think it's a nutritional deficiency. Some fertlizer should help. Also 'sometimes' I see yellow to brown discoloration in established growth when my Sagos are readying a flush.

Thanks. But I don't think a nutrient deficiency would cause a rapid decline like this, especially since it's been in the same environment for four years.

you said one of your sage plants is now dead in the same location where your cycas are located. both these plants can thrive in wide range of pH but will do best in moderately acidic soil. both these plants are very hardy in the right situation but this is not without boundary.

i do have a number of healthy cycas revoluta that are thriving where temperature gets over 35 degrees celsius with relatively high humidity. rarely are there seedlings having one of their fronds lie down due to number of problems.
i even forgot to water them for more than a month and their leaves lied down but with no yellowing.
soil is moderately alkaline and course-grained. they are watered 3 times a week on average but the smaller ones get more, and fertilized with complete slow release fertilizer high in N.

i could only assume that this is a persisting problem that developed gradually and the harsh sun light just made the problem more bold.

Sego Palms like sandy soil vs clay