This is a very interesting question, firstly I assume you mean the Liberal Democrat part as a opposed the Liberal Party, as they are both different.
Firstly, the Liberal Democrats have an array of different ideological beliefs within the party, this included centre-left Social Democrats to centre-right Liberal-Conservative. This lead to the first cause of the fall of the LibDems, the party membership was disconnected from the leadership. The base was slightly more centre-left, so they disagreed with the leaderships shift to form a coalition with the Conservative party. Many people in the LibDems such as Paddy Ashdown wanted to form a coalition with Labour. This was the first nail in the coffin.
The second was the failures of the LibDems in government, they had a massive U-turn on several key policies which infuriated the membership, such as increasing tuition fees. That made a lot of the young voters leave the party. They also failed to get significant meaningful reform, this is exemplified in how they failed to reform the upper chamber of Parliament which is unelected - House of Lords or to win a referendum on changing the voting system. This lead to the portraying of the idea of the LibDems just pandering to the Conservatives. This is limited to the extent that the LibDems did stand up against dangerous laws which the Conservatives where going to implement such as bombing Syria in 2013(Bashar Al-Assad).
Thirdly, I personally believe that the LibDems just faced failure due to the political climate of fear of a SNP-Labour Coalition. Which meant a lot of LibDem voters went to vote Tory or not vote at all, some of course switched to Labour. This is evident in how membership for the LibDems has significantly increased in the last couple of months with the so called #LibDemFightback.
Conclusively it is a series of factors which made the LibDems fall. However, I think they will be back again, liberalism is not dead in the UK at all. It just needs to find itself a strong leader and a new image.
Hope this answers your question.