Another reason may be prosodic, i.e. unstressed elements cannot occur in certain places. Bybee's Relevance hierarchy. Phonological Expressions of Morphologization. Vowels and Consonants deleted. Remaining forms draw from shrinking phonolog.
Most reductions are general, i.e. part of the overal phonolog.
Most occur at boundaries. Fusion is the end of the continuum of the cline. But you-guys seems to have become monomorphemic, as evidenced by youguys's as the possessive. What is the object Ergativity, i.e. who causes what. We also can see here certain hierarchies at work, esp.