The main causes were impurities - quite common, since this stuff was home manufactured - and OD. I personally knew few people who died from these two causes, and one could hear of many more from fellow travelers. Of course the safest way was to manufacture the stuff oneself, but that required fresh sap, which was available only in the summer (July to mid-September). A stash, if processed and stored properly, could technically last until the next summer, but it was usually used up by January for two reasons - increasingly larger quantities required to produce the same high, and sharing with fellow travelers. Producing sufficient quantities was also problematic because procuring sap was very labor intensive and carried non-trivial risk of being apprehended by farmers who owned the fields (the stuff was legal and produced for commercial purposes, so sapping of the sap by junkies produced substantial loss to the farmers).
Manufacturing from dried pods was a more involved operation that required more know how, proper equipment and chemicals (which were not easy to obtain) and could easily be botched. There was, however, a safer and more cost effective alternative of making a tincture and ingesting it - but it had certain drawbacks, such as awful taste (an effort was often required not to puke it out) and the high was not as good as one produced by IV injection. On the plus side, the effects did not wore out as quickly as ones from IV injections. This was a non-trivial advantage for two reasons: cost and the ability to go about daily business without the need of shooting up. Later, with the HIV infections through needle sharing that killed many more, tincture was decidedly a safer option, but one could still get HIV through sex (albeit opium dulls sex drive considerably).
As I understand IV drug use is the main source of HIV infections in EE and Russia.
Wojtek