Walaykum Salaam
There is a Hadith which reports that the Prophet (PBUH) stood while eating and/or drinking (forgot which or if it's both). The website I read this at (a website called "Muttaqun" (http://muttaqun.com) said that Muhammad (PBUH) did this to show that it isn't forbidden to stand and eat/drink, ESPECIALLY when one would have a hard time sitting.