Why was guru nanak father so annoyed with him?

Answer 1

Though this is historical question, and history says that Nanak's father remained angry with him when nanak initially learned and started preaching Gurmat thoughts, but turned liberal during his last times.

There was multiple reasons, of anger of father, few are here under:

1) Nanak's father was a orthodox Hindu. He was an Idol worshipper and follower of Santana Dharma and do all Karma Kaand which goes in Hindu Religion. Nanak had no interest in this faith. Nanak spoke against all Karmic ideals of Santana religion prevalent durin that time.

This was first reason of anger against Nanak to not to accept prevalent beliefs

2) Nanak had keen interest in spreading Gurmat Philosophy among people of different Sects of Hindus, Jains, Buddhists and Muslims. Due to this his family felt embarress when these people complaint to his father. Even Nanak got abused by people of all these sects.

This was second reason of anger that he say nanak to stop preaching philosophy.

3) In general, it is believed that Nanak did not find himself fit more in worldly services and inclined more towards spiritual one. Due to this nanak also heard a lot from his father. Famous one is to give food to hungry people by spending money given to him by father for business etc.

Once you will read hymns of Nanak in Adi Granth, you can easy to know that why his father might get angry with Nanak.

Answer 2

He change hi religion to sikshu