FillFactor: IndexOptimize script by Ola Hallengren
Ola's script will only change Fill Factor if you assign a value to the @FillFactor variable.

IF @FillFactor IS NOT NULL AND @CurrentIsPartition = 0 AND @CurrentIndexType IN(1,2,3,4) SET @CurrentCommand13 = @CurrentCommand13 + ', FILLFACTOR = ' + CAST(@FillFactor AS nvarchar)


Which means someone:


Modified the stored procedure to use 90 as a default value;
Ran it out-of-band from maintenance; or
Changed the values using a different script or the GUI.


You can script out the stored procedure to see if someone changed the default value, 



Or you can query the dbo.CommandLog table to see if a REBUILD command was run with a Fill Factor of 90.

SELECT *
FROM dbo.CommandLog AS cl
WHERE cl.CommandType = 'ALTER_INDEX'
AND cl.Command LIKE '%FILLFACTOR%'


If someone changed it by using a different script, or the GUI, that would be much more difficult to track down.