I should have read the article all the way through.
"'Recently, the weather in this part of the country has been relatively dry and water tables may have lowered. The strata at the site consist of about 4m (13ft) of sands, silts and gravels overlying chalk."
[...]
"In St Albans this chalk is covered with thin layers of silt, gravel and clay. As rainwater filters through these layers it becomes more acidic, dissolving the clay beneath. This process may have caused an underground chamber to collapse and in turn trigger a crater to form."
There is no mention in the article of concepts like "acid rain" but, with global CO2 elevated from 250 to 400+ parts per million you would expect a mild increase in rainwater acidity and long term affects below ground. However, the white cliffs on the Channel coast aren't exactly dissolving before our eyes, so it's clearly not that bad.
For what it's worth, the acidification of Scandinavian lakes was eventually proven to be caused not by acid rain from factory chimneys in the northern UK but by rainwater runoff through the layers of pine needles on their own forest floors - the reason why conifer plantations never have undergrowth is that needle drop destroys practically all competing plants by changing the pH of the soil.