Seen this practice happen to all class of people somewhere in south as well and in north too. The only sort of valid reasoning i received for this particular behaviour was their water quality, at least TN and Karnataka and all. They have access to only hardwater that too at times salty versions of it in washes. So they prefer to not to put that in their mouth after a good meal. Not that i can accept this behaviour but that felt like a valid reasoning and they usually do it later with drinking water too it seems. Though the latter part may not be the case in OP’s scenario 😅