A major source of the large volume of these trades is likely to be asset managers seeking to rebalance their portfolios in line with the indices they track that incorporate the London fix in their calculation.
One way to minimise this tracking error between their portfolio and the foreign exchange component of the index they are tracking is obviously to execute at the fixing price itself. Orders for these trades and other fixing trades are provided by the client to the FX dealers ahead of the fix (often this time is quite short).