so just to give you a little insight. I used to train people on
SEO.
What you experienced is actually very normal. Moreso, it is a great sign that G is testing the validity of your site for a given KW. It is healthy for a domain to pop in and out of a KW, also Up and Down. This usually occurs within the first 3 months after indexing.
However the success of the bounce can be affected by link building velocity. Keeping a steady link build will increase the likeliness and timeframe that it takes to keep the good rank spot.
Good Luck!