When Pinkie Pie wet herself, that made Fluttershy do the same, which in turn made Rarity wet herself. By the time Pinkie was able to cheer Fluttershy up, Rainbow Dash and Applejack, who had drank just as much but usually had strong bladders, got too excited seeing Fluttershy and Rarity respectively peeing themselves, so they lost control as well. It was a little embarrassing, but there was something about sharing this moment with Fluttershy and Rarity.
 
All equally wet, the girls shared a laugh before walking to Carousel Boutique’s showers, their boots making squelching noises with each step for they were full of pee. They exited the showers soaking wet, which was nothing new to them as they’d sometimes get caught in the rain on Mondays, and feeling much better.
 
The last pic in this wetting series, and you’ll notice I combined different screenshots for their positions.