Guys on sets of TV shows tend to have fun with each other. David Ortiz decided he wanted to have fun at the expense of Frank Thomas. It was during the ALDS preview show when the Astros were getting set to take on the Rays when Thomas walked away from his cup of water and Ortiz took matters into his own hands.
Mixing water and vodka doesn't really taste good and you can tell that Thomas wasn't a fan of it.
At least Thomas had a sense of humor about it.