Google measures the average age of your pages. When you start adding pages on a daily basis this will change the average age of your pages. The average age of your pages is compared to the average age of pages in your market (ie. keyword(s)). Google decides for each market what the upper limit and lower limit is for the average age of your pages. when you´re outside these limits, you don't rank at all. (google roller coaster effect).
When you started adding pages to your site, the average started dropping. Now, you can imagine that 1 new page on 50 existing pages doesn't have much effect on the average age. But after 50 days of adding 1 page per day, the average age has gone down a lot. Result may be you get higher rankings because you were within the limits. Another 50 days later the average age of your pages may have gone down so much your pages are now way too young.
So don't do accessive new page adding when the standard in your market is different.
Of course there is much much more to the algorithms than this one single factor. Google keeps track of many things. Imagine for example that all those new pages didn't attract any new links,... then what happens? Does it make sense that all that new content does not attract any new links? What does that say about the quality of that content? Maybe its not as good as it you think it is.
One of the biggest mistakes you can make in
SEO is to focus on 1 factor only. Getting obsessed with one factor makes you forget about all the other factors and often the quality of the work isn't that good because quantity gets overvalued while quality is much more important. And Google bases quality for a big part on links.
Here too, quality is not based on how many links you have. Who links to you is as important, if not more. A higher pagerank is nice but here too, if you get obsessed with pagerank, the quality will suffer and again you will focus more on quantity than on quality.
Try to understand what makes your website better for your audience and the links will come and when you know your audience, you will find that creating quality content is easy.
When I say audience, I do not mean specifically your customers. Think of all the people that may be interested in what they can (could) find in your website. Very few customers will link to your website, so perhaps it is a good thing to figure out who else you can attract, even if it means they´re not the people that are interested directly in buying your services or products.