I sing in a pub with the same group of people, have been doing it for most of 10 years now. And these charming, otherwise fairly restrained people, put a bumpity-bump (tiddly-pom to you Brits) sound on the end of the first (and other)lines of practically anything upbeat and 6/8 (and maybe 3/4 too). The sound is made with silverware on the glasses, hands on the table, salt shakers smacked together or practically anything else that comes to hand. Sometimes it gets rowdy enough that the servers come in and clear our table of breakables.
I have no idea who started this, or why it happens. It's like, punctuation.
I suspect that the thumping of the mugs or pints in "the Wild Rover" grew out of either rowdy audience involvement, or band members encouraging that. I can recall singing it in the Starry Plough in Berkeley in the '70s and encouraging the drinkers to slap their mugs on the table tops. Spills quite a bit of beer, that way. So perhaps the mug pounding deteriorated (?) into mere clapping.
Go figure.
Perhaps it's just one of those cultural group phenomena (like the wave) prompted by the urge to be drunk and silly together in a demonstrable way.
Blessings,
Barbara