I am not sure I understand what you mean, that website has a total of 35k pages submitted through sitemap to GWT, of which only 8k are indexed. The total number of pages indexed have always been slowly increasing through time, it moved from 6k to 8k in the last couple of months, slowly with no spikes.
That's not the total number of pages served by the site, since dynamics search results page amount to around 150k total pages, we do not submit all of them in the sitemap on purpose, and GWT shows 70k pages as the total number of indexed pages.
I analyzed Google crawler activity through server logs in the past, it does pick a set of (apparently) random pages every night and does crawl them. I actually never analyzed what percentage of those pages are in the sitemap or not.
Internal link structure was built on purpose to try to favor ranking of pages we considered more important.
The point is we didn't change anything in the website structure recently. User generated content have been lowering duplicate pages count, slowly, through time, without any recent spike. We have a PR campaign which is increasing backlinks with an average rate of around 3 links per week, and we didn't have any high DA backlinks appearing in the last few weeks.
So I am wondering what made google crawler start crawling much more pages per day.