The word tiras when used in the sense of 'disappearance' is called gati when in composition with a verb.,
As तिरोभूय, तिरोभूतम् and यत् तिरोभवति why do we say when meaning disappearance? Observe तिरो भूत्वास्थितः i.e., standing apart.,
तरो ऽन्तर्धौ
Adhyāya 1 · Pāda 4 · Rule 71