The groundings are happening as part of the investigation into the poisoning of ex-KGB man Alexander Litvinenko, and the two events might be related. Of course, the risk to anyone on board the planes was probably next to nothing, but people who flew on affected flights (a list can be found here)
From BA's website:
British Airways has been advised that three of its Boeing 767 short haul aircraft have been identified by the UK government as part of the investigation into the death of Alexander Litvinenko.
The airline was contacted last night (Tuesday, November 28) by the government. It has taken the three B767s out of service to enable forensic examination to be carried out.
The initial results of the forensic tests, which was confirmed late this afternoon, has shown very low traces of a radioactive substance onboard two of the three aircraft.
British Airways has been advised that this investigation is confined solely to these three B767 aircraft, which will remain out of service until further notice.
British Airways understands that from advice it has been given that the risk to public health is low.
The airline is in the process of making contact with customers who have travelled on flights operated by these aircraft, which operate within Europe.