Haven't done a ton of research yet, but think the answer is the same as in Array dereference in foreach(): map probably provides lvalue context to the list it is iterating over (because it allows modification of the original items of the list via $_), and autovivification happens in lvalue context. Further somewhat related reading:

• Autovivification Oddity
• Why doesn't this die with "Can't use an undefined value as an ARRAY reference"?"

Update: stevieb beat me to it :-)