Rumour has it that it's been caused by a ransomware attack (although Garmin hasn't commented publicly) - I can't think of much else that would cause such a long outage.
I thought the same. Especially the fact that their webservices as well as their callcenter and other support systems are down screams ransomware attack.
I'm sure we'll see a long and well funded security department after this incident at other companies /s