Thanks for posting and welcome to the community, I would be happy to help with this.
What you will be finding is that most commercials on YouTube are mixed to be louder than the video you have been watching. The reason they do this is so the commercial grabs your attention when it starts playing. The advertisers think that by being louder you will be more likely to notice their products and less able to ignore the commercials.
This technique is also used on TV broadcasts as well but to a lesser extent as there are restrictions on TV broadcasts as to how loud they are allowed to go.
You will find that if you listen to YouTube through your TV speakers rather than the soundbar the volume will also spike when a commercial is playing, it's sadly something the advertisers are unlikely to stop doing while they think it works.
I hope this helps explain what is happening. Let me know if you have any other questions!