Quatchgrass is a variant form, of minor importance, of the common name for various grasses, especially Agropyron repens.
This is only one of several onomatopoetic names for the same thing.
Others include couchgrass, quitchgrass, quackgrass, and twitchgrass, or just couch or quitch.
This last is the oldest known form, and it seems to be related to older forms of quick.
From this, we assume that the name was given to show that this weed grows and spreads quickly unless the farmer is on his toes and roots it out promptly.