、。
So that's why everything ends up weighing down on Mai's shoulders.
Literal
For-that-reason, all [subject-が] Mai-san [gen-の] shoulder [on-に] weigh-come [nominalizer-こと] becomes [explanation-訳だ].
それゆえに is a formal/literary causal connective ('hence, therefore'), a step up from だから or なので. 圧し掛かる (おしかかる) — 'to press down on, to bear heavily on' — is used figuratively for pressure or responsibility. The chained ~てくる signals the pressure coming toward/into her sphere. ことになる packages the whole thing into 'it comes about that...,' and 訳だ ('that's why, that's the reason') closes by framing the preceding clause as the resolved explanation of something the listener already suspects.