That drop for the New Democrats is actually deceptive. If we look only at the last set of polls from EKOS and Forum, the two firms in the field in July, we see that the NDP has actually gained when comparing apples to apples:
The Conservatives were ahead in Alberta and the Prairies, and were second in Ontario and Atlantic Canada. They were tied for second in British Columbia with the NDP, and were fourth in Quebec.
The NDP was only tied for second in British Columbia and was third in every other region of the country. That meant the Bloc had displaced the NDP for runner-up status, the first time that has happened under Thomas Mulcair's leadership.
The polling data is a little too thin to draw any significant conclusions. But it does seem that the Liberals are coming down from their highs after Justin Trudeau's leadership victory and that both the Tories and NDP are winning back some of that lost support. Nevertheless, the Liberals remain ahead and the Conservatives and NDP are still showing signs of weakness in important parts of the country. The rebound of the Bloc is not likely to be anything definitive but they may have become the 'none of the above' option that will gather support whenever the Liberals and NDP fail to impress. Whether that support would show up at the ballot box, however, is another question entirely - and the answer will be vital to the hopes of both Trudeau and Mulcair in 2015.