CM

The link at the bottom provides one of the better and more comprehensive guides for identifying tomato problems and diseases. You have to click on the links to really see what's there, but you'll at least be able to eliminate a lot of possibles (Sherlock Holmes-like) and the dig into what's left. There is a section of pictures of diseases (strangely called the "gallery", though I guess it is).

Here is a link that might be useful: Cornell University -- Tomato disease identification

The odds don't favor 'disease' in young seedlings. The most likely cause (without seeing a photo) is root rot from too much water. That results in compacted, saturated soil. Most way over-water their seedlings even tho they don't 'think' they are and it is a hard habit to break.

Next most likely is simply not enough light. Photosynthesis produces the energy the plants need to support life and growth. If the energy to stay alive doesn't exist or the roots can't support the top growth, the plant begins to shed in an effort to stay alive.

Dave

It's definitely not root rot (in fact, the soil is dry and needs to be watered very soon)

Also not a light issue, because all the other seedlings under the same light are doing fine.

I will check the disease gallery and see what I can find.

In the meantime, should I move possibly infected plants away from the others - could the disease be contagious or will it be OK leaving them in?
I also thought it might be more of a varietal trait because it's only happened to two Indian Stripe seedlings. Maybe they are more prone to losing their old leaves. I will keep a close eye on them and not over-water them.

Looks like it might be powdery mildew, but possibly bacterial canker (I hope not!)

I just picked off a leaflet that was just beginning to show some spots. I'll see how it all goes. So far, nothing other than Indian Stripe apparently has the problem. I'm hoping it's not a serious problem.