Why was Hagia Sophia reinforced from the outside after its dome collapses?

The Hagia Sophia was reinforced from the outside primarily to provide structural stability following partial collapses of its dome caused by earthquakes.12

After the initial 6th-century construction, the dome suffered a partial collapse in 558 CE due to seismic activity. Following this and subsequent partial collapses, the dome was rebuilt to a smaller scale, and the entire church was reinforced from the outside to ensure the integrity of the massive structure. These external supports were essential to counteract the immense lateral forces exerted by the dome and to preserve the edifice, which remains one of the world's most significant architectural monuments.12