why does it start to swap when there are still plenty of memory?

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           125G         16G        105G         26M        3.3G        107G
Swap:          8.0G        1.9G        6.1G

I am running a program to process a big file and this is what I am seeing. I don’t understand why it starts to swap a lot when there are still 105G available.

My system is Centos 7