Long mergeout every day

Hi guys,
I have problem witch mergeout in my vertica cluster. Once a day start big mergeout at the same time and working at 3-4 hours. I don't understand why it happens?

Statistic mergeout:

 DATE_TRUNC           min_ros_gb  max_ros_gb  sum_ros_gb  count_merg  
 -------------------  ----------  ----------  ----------  ----------  
 2018-02-28 13:00:00  42.72       43.12       171.96      4           
 2018-02-28 12:00:00  26.95       43.25       506.74      14          
 2018-02-28 11:00:00  27.01       27.24       162.85      6           
 2018-02-28 05:00:00  129.69      129.82      259.51      2           
 2018-02-28 04:00:00  129.53      129.79      1296.59     10      
 2018-02-28 03:00:00  84.69       84.92       1018.06     12
 2018-02-27 23:00:00  54.33       54.33       54.33       1           
 2018-02-27 22:00:00  54.31       54.61       326.70      6           
 2018-02-27 21:00:00  35.71       54.66       482.44      11          
 2018-02-27 20:00:00  36.00       36.21       216.71      6           
 2018-02-27 14:00:00  39.61       39.68       79.29       2           
 2018-02-27 13:00:00  39.54       39.76       158.68      4           
 2018-02-27 12:00:00  26.28       41.05       199.38      7                   

Every day at 3.00 - 6.00 running big mergeout. Objects are the same - two big flex table.
How can I fix / optimize it?