Global blocks are issued when a user violates the Terms of Use. In general, a user is globally blocked for:
underage users reported to Fandom Staff
Inappropriate posts (NSFW stuff, threats, etc)
Doxxing other users
Discrimination (Racism, LGBTphobia, ableism, etc)
Impersonating other users
Large scale and/or persistent vandalism across several sites
advertisement spam
Other ToU violations
Problems in one community don't necessarily mean that a user should be blocked across all of Fandom, but when a user does something serious enough that they can be considered disruptive to multiple communities or they're underage (because of COPPA, which is a law across multiple countries), they'll get blocked across Fandom, and not just one community.