That is due to software release cycles.

For more details, please see:
http://www.capitalismlab.com/digital-age-dlc/software-report/

Indicators of New Releases
As shown on the above screenshot, yellow lines on the sale graphs denote new releases. As you will notice, whenever there is a new release, the sales go up. Apparently, it is important that a software company keep rolling out new versions on a regular basis.

My company's software market share is number one. But revenue is always up and down in 6 months cycle. Is it natural in game function??