Pictures would be great. Here in Raleigh the Tulip poplars are about halfway leafed out here and none have started to bloom. May be too early for you to know quite yet. If you are able to post a picture that would be great. Thanks and good luck!

You've had a bad drought there starting last summer. Tuliptrees aren't particularly drought-tolerant, so perhaps some that were already stressed for some other reason gave up? Hard to say.

Could it be verticillium wilt? I lost two sugar maples to this. Thier symptoms and sudden death were exactly like you describe. Tulip trees are a susceptible host species. Weak trees succumb quickly.

If you choose to replant in the site and the cause of the tulip trees' death is verticillum, make sure you choose wilt-resistant trees; it stays in the soil for a long time.

Here is a link that might be useful: Verticillum Fact Sheet

Has there been any kind of construction or trenching activity in recent years?

I am more inclined to think the severe drought in the western part of the state, combined with growing next to a road, which is a stressor in itself, may have been the final straw. If the tree(s) came down recently enough, take some branch/leaf samples, as well as soil samples (keep them clean - new ziploc bags? - and not mixed with your lawn/garden soil, both to be sure what THEY have, and to protect your garden if there is a problem) to your county extension office, and ask the agent if s/he can identify any reason for the death.

While it may appear to you that there is "no reason" for the tree(s) to die, unfortunately, you can't know all the factors involved - there may have been an underlying weakness that the drought exacerbated. Trees do have a finite lifespan, and it may be that this tree, and the others, have reached the end of theirs - which is not an answer, I know.

Hi - Shannon here.
I couldn't get around to posting pictures yet, but I can add details. The tree fell over from the roots and left a hole about 20" deep and about as wide. Despite the tree being on top of a steep slope, most of the roots didn't come up with it - just a few big ones right around the trunk. The exposed roots looked very sick. Well, I guess a better description would be to say they were dead. Dry, rotten, and no sign of life. The dead tree is still on the ground and no stump sprouts have emerged. So here's the rundown: Tiny leaves, lots of lichen, and rotten/dying roots. It was really doomed.
The second tree doesn't look nearly as bad as the dead one before it fell - just smaller leaves and no significant lichen.

Roots killed by decay fungi cannot send enough water up the tree to support leaf development, and do not provide a firm anchorage. Your undersized leaves and broken punky roots strongly suggest root rot as cause of death. Other tuliptrees trees nearby are probably at risk -- keep an eye on them for signs of wilting this year or small leaves next year.