I'll let the real experts chime in, but IME it's not uncommon for tomatoes' bottom leaves to die off as the plant grows. In this case, my guess is that these babies are ready for transplanting and my be a little root bound. Plants do sometimes shed leaves to maintain balance with the size of their root systems,

Otherwise, they look very healthy to me! :-)

Just to add that when you do transplant them, it's quite common to bury tomatoes deeply, even up to or beyond these bottom leaves. You can strip off the dead ones then, and the plant will grow roots there from the buried stem. This larger root system can then support a nice big plant.

Have you added fertilizer?

If not, please start.

They also need more light.

Just double checking that you did poke drainage holes in the bottom of the cups.