TL;DR Film production companies stonewalled and stalled negotiations, even no-showing many scheduled meetings until it was obvious they had no intention of signing a new contract. That's what prompted the strike.
The issue here is that the current SAG labor contract was signed in 1960 and has received only incremental modifications. The actor's union is saying it doesn't deal substantively with issues like profits from digital streaming, likeness ownership, and AI generated voiceovers and personas.
The production companies are saying greedy millionaires like Harrison Ford and Jennifer Lawrence just want buckets of money to add to their substantial wealth. The actors are saying they're fighting for Harrison Ford the carpenter and Jennifer Lawrence the high school dropout, the people they would have been if they hadn't become A-list stars. That is the bulk of their membership, the millions that do tv commercials and voiceovers and stunts that no one knows.
Those members are working class people who are not famous or household names, their only leverage is from the union that works for them to get fair working conditions.
It's also come out recently that the production companies have no intention of settling the writer's strike anytime soon. Their current strategy is apparently to keep it going until the fall when writers are usually paid for pilot season. Without pilot season the writers may begin to lose apartments and houses and face hard choices for healthcare and education for their families. The production companies feel that a taste of homelessness will make them more flexible in negotiations. Nice, right?