why the ruamel.yaml automatically add anchors and aliases?

Aliases are the way YAML can dump shared data, in your case the list/sequence ['flat']

The anchor and aliases mechanism is necessary to be able to dump recursive data structures, i.e. ones that are directly or indirectly self-referential, like:

data = dict(a=1)
data['b'] = data

The above is something that simple serializing languages, such as JSON, cannot deal with.

Since you just have a shared data structure that is not self-referential, you can either assign copies:

    port_dict["networkType"] = sriov_seg_type.copy()

or tell the representer to ignore aliases:

import sys
import ruamel.yaml
from collections import OrderedDict

ruamel.yaml.representer.RoundTripRepresenter.ignore_aliases = lambda x, y: True

yaml = ruamel.yaml.YAML()


data = []
sriov_port = ['P1', 'P2', 'P3', 'P4']
sriov_seg_type = ["flat"]
for port in sriov_port:
  port_dict = dict()
  port_dict["name"] = port
  port_dict["mtu"] = 9000
  port_dict["networkType"] = sriov_seg_type
  data.append(port_dict)

yaml.dump(data, sys.stdout)

which gives:

- name: P1
  mtu: 9000
  networkType:
  - flat
- name: P2
  mtu: 9000
  networkType:
  - flat
- name: P3
  mtu: 9000
  networkType:
  - flat
- name: P4
  mtu: 9000
  networkType:
  - flat