Why does dropping a MVIEW trigger a full refresh?
After running some tests in an isolated environment it seems that Raj is right: the refresh background job that I saw while waiting for the DROP wasn't triggered by the DROP. As far as I can tell this was caused by the refresh settings provided during the creation of the mview. 

It seems that 

refresh start with trunc(sysdate) + 1/8  next sysdate + interval '8' hour


together with the implicit build immediate causes the mview to be refreshed during the creation and then again immediately after the creation has finished. 

Apparently start with trunc(sysdate) + 1/8 causes this. When running 

create materialized view foobar 
   BUILD DEFERRED
   refresh start with trunc(sysdate) + 1/8  next sysdate + interval '8' hour
as
select ...
from ...;
The create statement finishes quickly and when I then look into the active sessions I do see one background session that does the refresh.