What is the underlying cause of austerity?
Capitalism bred leo-liberalism. Neo-liberalism bred neo-liberal austerity. One of the key components of neo-liberalism using governmental austerity is to reduce/cut public expenditures for social services that include:
All forms of " social welfare" to aid the poor and eliminate poverty. (Of course, corporate welfare in the forms of subsidies, tax loopholes, and government contracts are acceptable.)
Cutting governmental expenditures for social programs eases pressure on the government's obligation to adequately tax the accumulated capital of the wealthy and huge multi-national corporations.
The ultimate result of government-forced austerity upon the working class and the poor is that it allows the capitalist ruling class to retain more (and continue to accumulate more) of their capital (wealth).
A capitalist system, whose prime directive is the production of capital, will work constantly to refine and improve its ability to do just that. It will continue until it is stopped by an external force of some kind, or it collapses under its own weight.