They're basically a clumsy person... that's probably what's coming back to bite them.
Literal
Fundamentally, clumsy person [reason-だから]... that [subject-が] is-cursing [explanatory-んだろう].
祟る (written here with the visually similar kanji 崇 — a common mix-up, since 祟 and 崇 differ by only one stroke) means 'to haunt, to cause bad consequences.' Colloquially, それが祟っている = 'that's what's coming back to bite them,' applying the supernatural 'curse' metaphor to mundane cause-and-effect. The trailing dots and んだろう create a musing, thinking-aloud tone.