Some of these are inspired by the answers to question I asked on the conlang mailing list:
* Erosion (and all other forms mutation) of causes endings to drop off or for inflections to become identical to others.
* Inflected forms split into different words, maybe morphed beyond recognition and shifted to new semantic ground, for example, a past tense inflected form becoming the particle or adverb indicating the past.
* Ending just drop off leaving a bare stem (unlikely to happen without a lot of intervening steps)
* Endings split off to form particles and clitics (which tend to be phrase level, less bound to any particular word)
* Boiling down to one form, maybe a neutral one, like the dictionary form (seems unlikely), or 1st person present.
These sort of processes happened in recent times in English and Swedish, so I suppose additional ideas could be found by studying the recent evolution of those languages.