One reason is the fact that YouTube/Google is looking to buy. Any time a large company buys another, the purchase is inspected incredibly closely to see if both parties meet certain certifications and requirements. To continue with the requirements part, as a company grows and hits a certain point in both revenue and business size, they are asked to be compliant in many areas.
I’ve seen this before with companies who process credit cards, store information, or facilitate the exchange of money in general. They are required to show that security is a top priority. Originally they have fairly easy security ‘asks’ of them. Encryption, database security, etc. But as the company grows and deals with larger clients, those clients ask a company to be further certified (PCI DSS for example).
In Twitch’s case (this is just me guessing), I imagine during the YouTube purchase talks they were asked certain questions about how they deal with the streaming of copyrighted content. Those answers weren’t good enough to be seen as ‘compliant.’ In order to reach compliance they had to take measures (as seen with the recent changes) in a timely manner. I think this is the main reason the changes were rushed live and not discussed with the public first. It was a rocky implementation, but would have happened eventually as Twitch continues to grow.