On 9th of December last year bounce rate of this mobile website jumped from 20 percents to 40%s. Does anyone have an idea where should I start looking what happened on that day?
I had a similar experience. The bounce rate of one of my sites suddenly went down to 20%, then to 0%. I first thaught maybe people are just that interested in our site.
In the end what I found was that we had a duplicate Google Analytics tracking code in the head section. The tracking code was already inserted via the Wordpress theme and then I added it again via a Wordpress plugin.
Why it went from 40, to 20, to 0, I don't know. But what I do know is that de-duplicating the Analytics tracking code got the bounce rate back to normal levels.
20% is a very low bounce rate. I know it depends on the website, however, as a general benchmark, I would trust that the 40% is more true to life. Particularly as your bounce rate dropped considerably in April 2015. It is likely that you implemented something in April 2015 such as some event tracking which was not set to non-interaction which considerably impacted your site in April 2015 and which has now been removed? I would go to Behaviour > Events > Top Events to see if there was a considerable difference in an event between December 9th and December 8th last year