A lot of it probably.

They have a breakdown of the data on their blog. If you scroll down to album releases by date, you can see a very sharp uptick in releases, with around ~2 million albums in 2019 to ~11 million albums in 2024.

They even make a comment on it likely being inflated by AI:

If we group albums by release year, we see that more and more new music is added to Spotify, a lot of it likely automatically generated: […] The amount of procedurally and AI generated content makes it hard to find what is actually valuable.