Herman, Leah's brother came to the UK in 1899 or 1900 (according to a police report of an interview with him in his Australian naturalization file). When did Leah come?
I suspect that we are going to have to look at the history of the Jew people in Europe and the Netherlands for likely reasons for emigration. It might just be that Herman and Leah were part of a more widespread trend to move at this time.
I have done no research on this yet, but specific issues like antisemitism and more society-wide ones like unemployment or an economic depression are the usual suspects.