Your guess would be correct! Most, if not all, traders use Bitcoin as their main trade medium. By that I mean they usually convert the altcoin back to Bitcoin when they are finished trading. This means that altcoin drops and Bitcoin price increases.

Another reason could be a mass migration of people from an altcoin to Bitcoin. For example, when Ethereum forked, Bitcoin saw a price jump as many people changed their assets in Ethereum to Bitcoin to avoid the fork. The same happened when all exchanges froze Ethereum transactions in July. Everyone switched to Bitcoin after seeing that Ethereum was not exactly reliable. Hence, Ethereum saw a price drop and Bitcoin, a slight price increase. The pattern occurs for Ethereum and Ethereum altcoins.

However, these are not the only factors involved in determining the price of Bitcoin. There are other elements prevalent in determining Bitcoin price. But for now, I think I answered your question.