Run something like this to see your backup history and see if there are full backups running at other times. You can add DIFF and LOG to get more results:

SELECT a.server_name as 'Server',

a.database_name as 'Database',

convert(varchar(25),a.backup_start_date,100) AS 'Start Date',

convert(varchar(25),a.backup_finish_date,100) AS 'Finish Date',

DATENAME(weekday, a.backup_finish_date) AS 'Day' ,

datediff(minute, a.backup_start_date, a.backup_finish_date) as 'Mins' ,

cast(cast(datediff(minute, a.backup_start_date, a.backup_finish_date)

as decimal (8,3))/60 as decimal (8,1)) as 'Hours' ,

case

when datediff(minute, a.backup_start_date, a.backup_finish_date) > 0

then cast(ceiling(a.backup_size /1048576) / datediff(minute, a.backup_start_date, a.backup_finish_date) as decimal (8,1))

else 0

end as 'Meg/Min',

ceiling(a.backup_size /1048576) as 'Size Meg' ,--cast((a.backup_size /1048576) as decimal (9,2)) as 'Size Meg' ,

cast((a.backup_size /1073741824) as decimal (9,2)) as 'Gig', -- div by 1073741824 to get gig

a.user_name,a.backup_size as 'Raw Size'

FROM msdb.dbo.backupset a

join msdb.dbo.backupset b on a.server_name = b.server_name and a.database_name = b.database_name

WHERE a.type = 'D' and b.type = 'D' /*D=Full*/ AND a.backup_start_date > getdate() -40 -- Last X days

group by a.server_name, a.database_name, a.backup_start_date, a.backup_finish_date, a.backup_size, a.user_name

order by a.server_name, a.database_name, a.backup_start_date desc