Possible reasons I can think of are:
1) If soil is modelled as springs, check the soil stiffness under piles. For indirect checking check if the slab making excessive deflections under column base.
2) If soil is not modelled with springs and instead supports are placed under the footings and...
The soil condition cannot differ such much to require a pile under some columns and strip footing (not shown assuming) 5 meter away from the piles. Probably there is higher forces on the columns with piles. This should be the reason. Otherwise, you need to provide more information.