Apart from reaching a new ATH, bitcoin is also regaining dominance vs other alts. It was halved between March and June and bitcoin has been slowly regaining it but lately seems to have received a boost.

Source: (
https://coinmarketcap.com/charts/#dominance-percentage)
Any ideas about why this is happening?
I dont think Bitcoin ever lose its dominance. Even though a lot of alts are rising Bitcoin will still the foundation of them. Bitcoin never lose its dominance in reality it's just that these new alts caught the attention of the people in the start but only few of them maintain their greatness unlike Bitcoin. Bitcoin is the most powerful of all cryptos.
Bitcoin has been dominated and is dominating from its vary beginning and there is no another currency that can compete with Bitcoin. The reason of the popularity and reputation of Bitcoin is due to its features and functions and there is no another currency that has features and functions like the Bitcoin that is why Bitcoin is the choice of people and has dominancy over the market.