My belief is that it's a combination of:

• The management and other tenants of their multi-tenant office building got upset at people protesting outside
• Karpeles got freaked out by being confronted
• Karpeles intends to deliver further bad news and sees things escalating in the real world

The new address is a mailbox address behind a security gate. No one can get in. Even if someone got in, no MtGox staff would be there. No other company there expects visitors in person so no one cares how big of a neckbeard army accumulates outside.

It is bad news.