Why is fatiquer conjugated with être instead of avoir

Glen, I think you have possibly already answered your own question.

 ‘Cette randonnée m’a fatigué. Je suis vraiment très fatigué! ‘ 

The first verb is the verb fatiguer, conjugated with avoir using the past participle fatigué, and the second is être followed by the adjective (adjectival past participle) ‘fatigué’.

Fatiguer, ‘to tire/to fatigue‘, conjugates with avoir in compound tenses, but to describe being in a state of tiredness or fatigue, it is être fatigué, in the appropriate tense or mood.

https://www.larousse.fr/dictionnaires/francais/fatiguer/32978

I don't know which sentence you refer to, but I'll just take a guess:

Ne soyez pas fatigués! -- Don't be tired!

This is the imperative, and soyez is the imperative (2nd person plural) of être. It looks like the subjunctive but it isn't.

‘ne soyez ….’ is also the ne explétif followed by the ‘vous’ form of present subjunctive of être , which is what the post in Glen’s question links to.