The death of the composer is the most common cause for a symphony to be left unfinished, but it can also be abandoned due to lack of progress, frustration or changes in style, among other issues.
Even when a symphony is complete, parts of it may be lost later on, thus technically making the work "unfinished" even if the composer actually finished it.
Sketches can range from a few notes and motives, to complete works in short score or unorchestrated manuscripts.
Parts from previous compositions may be reused to complete the work, or the material may be rearranged without adding new music.
Some composers expressed their desire for these fragments to be destroyed or hidden from public view.