All except 1919 and 1926 were election years. There was a clear pattern for MPs and former MPs to defect just before or just after elections. The 1918 election had only just taken place in December, so 1919 saw much of the ensuing political fallout.
Another factor which increased the numbers of defections in the past was a rapid fluctuation in party fortunes. The first Labour government was formed in 1924 and the second in 1929. 1931 saw the formation of the National Government, the Labour Party expelling their former leader, Ramsay MacDonald and the Liberal Party divided over support for the National Government. 1926 saw Lloyd George succeed Asquith as Liberal Party leader. 1981 was also a significant year seeing the SDP attracting 28 MPs away from the Labour Party, although this was a party split (more akin to the Liberal Unionists) rather than a series of individual defections.
There also seems to be one other factor which plays a part - the ending of coalition governments. The Lloyd George Liberal/Conservative coalition government ended in 1922.
2015 could very well be one of those years when all of these factors come together.
Defectors and the Liberal Party 1910 to 2010 - a study of interparty relations published by Manchester University Press charts the fluctuating numbers of defections and accounts for all of the 122 defections of MPs and former MPs to and from the Liberal Party over the century to 2010.