That said, it’s likely the sealing wasn’t as permanent as previously believed, and in the time that’s passed between the first movie and Pacific Rim: Uprising, the seal has weakened to the point that the Kaiju can break through.
This is the most logical option given that it wouldn’t feel like a Pacific Rim movie without Kaiju terrorizing humanity, but who knows, maybe the Uprising part actually refers to the Jaeger pilots deciding to unionize because they don’t feel like they’re being paid enough.
In case you haven’t watched Pacific Rim in a while, the movie ended with the Breach, the interdimensional portal that the Kaiju were coming through, being sealed.
The answer that immediately comes to mind is the return of the Kaiju.