What Weakness in the Abbasid Empire was revealed when Tahir of Khurasan

The Tahirid dynasty (Persian : طاهریان ‎‎) was a dynasty, of Persian[3] dihqan[4] origin, that governed the Abbasid province of Khorasan from 821 to 873 and the city of Baghdad from 820 until 891. The dynasty was founded by Tahir ibn Husayn, a leading general in the service of the Abbasid caliph al-Ma'mun. Their capital in Khorasan was initially located at Merv but was later moved to Nishapur. The Tahirids enjoyed a high degree of autonomy in their governance of Khorasan, but they remained subject to the Abbasid caliphate and were not independent rulers. [3]