Yeah, arbitrary vandalism was the first option I considered. Can't rule it out, although whoever did this seems to have shown more caution/coordination than you'd expect from a couple of drunk idiots taking potshots at street signs.
Compare e.g. the incident where a disgruntled worker decided to flood an area with raw sewage through false radio commands (https://www.mitre.org/sites/default/files/pdf/08_1145.pdf)
If it was an insider, it was probably someone with a workplace grievance. Maybe someone who had raised concerns about security and wanted to prove himself correct, or just someone with a boss they disliked who wanted to make trouble.