The TIOBE Programming Community Index shows Java and C dramatically falling in popularity between late 2016 and mid 2017 before rebounding by mid 2018. What is the cause of the volatility?
Without access to the raw data or the underlying analytical methodology no one can hope to answer this question.
That being said there is a good balance of probability that the tiobe index faq section already contains the relevant answer:
Q: What happened to Java in April 2004? Did you change your methodology?
A: No, we did not change our methodology at that time. Google changed its methodology. They performed a general sweep action to get rid of all kinds of web sites that had been pushed up. As a consequence, there was a huge drop for languages such as Java and C++. In order to minimize such fluctuations in the future, we added two more search engines (MSN and Yahoo) a few months after this incident.
Essentially, the data collected from Google changed fundamentally, as such the index was unaware of or could not perceive the underlying reality.
Perhaps there were other factors but in understanding any sort of analytical system, always suspect the data source, and method of analysis first, reality a far distant second.