This is described in the anime "Blade Runner Black Out 2022", which serves as one of three prequels to Blade Runner 2049.
With the introduction of the Nexus8 line of replicants, humans became more violent toward them and started to hunt and kill them. A few of them tried to erase the database of replicants. Iggy, Trixie and Ren, the technician responsible for a nuclear warhead test, decide to attack the Tyrell Corporation server (Trixie and Iggy in a truck) that stores this data, and by exploding a nuclear warhead (Ren's responsibility) above LA they created an EMP that erased all electronic data stored in different places (like police or medical records).