Generally that is caused by damp-off - a wilt that attacks the base of the plant where it touches the ground. But it's unusual to have that happen to larger plants. Generally caused by over-watering, not enough sunlight (especially if plants are indoors) or dirty containers. But these are not frost hardy - aren't you still getting frost up your way?

Well, official frost-out, according to the woman at the nursery, is Memorial Day. But it's been warm the past few days. And I just think it's weird that two of the six are thriving, and they aren't even in the same pot. Others around them are wilting. I made the frost mistake with my impatiens, so I bought some more and am keeping them inside until Memorial Day. I'm still new at this, so I'm learning as I go.

Am I correct in reading your post line that you're in zone 4? If that is the case, no way should you be planting warm season annuals in the ground yet.

Yes, it may be warm during the day, but you have to remember the soil isn't thoroughly warm yet, and warm season annuals balk at cool soil. Also, it's cool/cold overnight. It can be in the high 60s/into 70s during the day but drop down to the 30s at night this time of year - not cold enough to frost but way too cold for annuals.

The exception would be if you're planting in containers that are close to the house (say, on the porch) - the sunlight during the day warms the soil in the containers (i.e. the soil is warmer than the ground soil), and the warmth from the house protects them on cold nights.

Most of us tend to jump the gun when it comes to planting out, but in my zone, which is at least a full zone warmer, we don't even consider planting out until middle of May, and even then have to keep an eye on the weater forecast.