Log messages regarding automatic vacuum and exclusive locks

Good morning. I'm seeing several of the following log messages each
morning (for example, there were five this morning, spaced
approximately one minute apart, with the closest interval between
messages being 44 seconds). They're occurring during a daily cycle of
deleting all rows from a table and then repopulating it from another
database; the first message came approximately seven minutes after the
process started and the last one came about a minute before the
process ended. There are approximately one million rows in the
database after repopulation.

LOG: automatic vacuum of table "x.y.z": could not (re)acquire
exclusive lock for truncate scan

I'm using default settings for autovacuum, with server version 9.2.4.

Some other smaller tables produce the same messages during the same
process, but apparently less frequently.

Any thoughts on the cause? Chapters 18 and 23 of the documentation
provided some hints, but nothing conclusive.

Thanks in advance,

Dominic Jones