What goes up like a rocket can just as easily come down like a rock.
Dunno why LCC's stock took the biggest percentage hit today, but with oil still about $62/bbl, airline stock speculators (hard to call any buyer of airline stock an "investor") may finally be realizing that huge profits may not be just around the corner. AMR, CO and B6 have seen some declines from their recent highs as well.
A less charitable explanation may be the realization that LCC has some real uphill challenges ahead of it: Sticking two sickly airlines together doesn't necessarily mean you've just created a competitive powerhouse. Maybe it means you've just created a much larger sickly airline. Let's be blunt: USAir was on the ropes prior to the merger and HP wasn't a Wall Street Darling, either. When LCC hit $40 the other day, its market cap exceeded that of AMR. My guess is that reality may be setting in.