Well, it's kind of hard to tell, but could it be transplant shock and they just needed time to adjust to being planted, and maybe more water right after they were planted? You mentioned the soil was on the dry side. Even if you water when you plant, depending on how much you soak them, if the surrounding soil is dry it can really pull that moisture away from the plants. Also, if you plant during a hot spell or on a hot day it can weaken them. You're right, marigolds are tough, but I have seen them wilt after transplant, especially if they were heavily rooted in the packs and just had a tiny bit of soil in the pack instead of coming from a 2" pot or something more generous.

I seriously doubt it was a fertilizer issue unless you got carried away, but that would be showing up in your other plants as well. Fertilizer issues show up as burn or other physiological damage, and not just wilt. Ph shouldn't be much of a problem for marigolds unless you have something extreme going on. In any event, I think either of those problems would take more time than overnight to show up.

You might want to have your soil tested if you aren't sure what the fertilizer might have done, just to avoid future problems on that score.

Good luck!