Hi Gaurav,
The behaviour you are seeing with the orientation of the exciters is expected.
If you don't specify the Exciter type while creating a yagiuda antenna, it takes the 'dipoleFolded' object as it's exciter and a 'dipoleFolded" exciter is a dipole folded antenna placed on the "xy" plane. This means that the length and width of the antenna is parallel to the xy-plane.
On the otherhand, When you specify exciter type to be a "dipole" then a strip dipole antenna is placed in the "yz" plane. This means that the length and width of the exciter antenna is parallel to the yz-plane.
You can adjust the tilt axis and tilt angle of the exciters by changing the "Tilt Axis" and "Tilt" properties of the exciter respectively.
To know more about the dipole and dipoleFolded antennas refer to the below documentation:
Hope this Helps!