You should not focus too much on the drop of your average position and the true measure of success of your website comes from your impressions, clicks, and total visitors. However, if you feel this position drop is something worth taking a closer look you should look at some things:
1. Did your competitors improve their content quality recently? You can try to make some searches for the keywords you are targeting and see what your competition is doing to achieve better positions than you.
2. Check your website for stale, old content that might have become less useful for your visitors.
3. Verify if the number of backlinks to your website is the same, or higher and if they are of the same quality, at least.
4. Last but not least, this could also be caused by a change in the Google algorithm.
Hope that helps and please do not hesitate to let us know if you need our assistance with anything else.