Hmm, you'd be surprised what a couple of quid here and there can do to a market, especially if the bets are all within a specific time-perimeter - like dominoes.Those odds seem very very short though, indicative of really heavy bets from quite sourced people.
If the odds are already short in a market like this, where information can really play a part, then two £50 bets could mean they really shorten the odds in the market.