It also seems to be solely on the back side of them, could it be that those leaves arnt getting enough light/sun?

When was the last time they were repotted? Organic potting mix breaks down over time and becomes denser and less chunky. After a while it becomes "dust" that may get compacted, muddy, and not drain properly.

It would help if you could lay the pot on the floor, slip the rootball out and see what's going on there. Is it wet? Are the roots firm or mushy?

Also, press on the trunk to feel if it's spongy next to to a discolored leaf.

The presence of fugus gnats may indicate overwatering but you should check the bottom of the pot and the roots.

Probably being kept too wet for this time of the year.

I checked the soil it was dry to the bone and the roots were growing through the bottom of the pot and they seemed strong. Not spongey or black. The one root was a fuzzy pinkish. The soil did seem very “dusty” as JoJo mentioned. I gave them both a good watering and poked some holes in the dirt woth wooden skewers to kind of break up the dirt bc it did seem really compacted but they’re still loosing leaves.

How long have you actually had them? The top leaves are still very erect which is a good sign.

From a horticulturist I can tell you they do not work well indoors as a houseplant. Unless you have a wall of big south/ west facing windows. This plant likes dry conditions. Ideally If you were to water it well ..flush the water through the soil once every 3 weeks and let it dry out.

On a side note. It is mis information when people are told to water any plant when the top 2- 3 inches of a pot is dry. This will lead to root rot. Do not do it LOL let it dry out.

The whiteish patches on the undersides of the leaves may be powdery mildew, which can be a problem for yucca. Unfortunately I don’t have experience successfully treating for powdery mildew, perhaps someone else can give advice there. The plant still looks good, however you are right that it should only be losing its oldest leaves, I think you do have a problem.

i have several yuccas . this plant is  one of the easiest plant to grow, it doesnt need any thing to do with it just from time to time water it let it dry sometimes forget to water it . and you can put it in a place with low light or high light or direct sun but not summer sun .  best place is near the window . 
in your case i think you should cut the older leaves becoz every leaves has age when their time finished the leaves starts to die . so before that time you cut them so the power will go to other or above leaves and starts to grow a new leaf faster . 
i have a very tall yucca since last 3 years iam looking after it when my father gifted me was around 2 metres tall but now its around 4-5 metres tall its like a tree now .