Path of Exile – How to Spawn Oshabi

But then some kind of explosion happens and I die.

It could be a Detonate Dead effect, some monsters explode after being killed (particularly those raised by a Stygian Revenant). Particularly in high-level areas these explosions can deal significant damage (I am pretty sure I got one-shotted as well a couple of times). An example can be seen in this brief video.

Looking at the official forum, it seems that you/we are not alone.

An effective mitigation is to stay away from corpses for a few seconds after killing them.

When you complete a Shaper-influenced map that is adjacent to an Elder-influenced map, the map you completed will usually flip from Shaper to Elder influence instead of returning to its default status. The converse is also true -- an Elder-influenced map will usually flip to Shaper influence on completion if it's adjacent to another Shaper-influenced map. This is the "ping-pong" effect that the post is referring to.

Shaper- and Elder-influenced maps have added monster packs and can drop Elder/Shaper rare items. These items can be quite valuable, as they can have mods which are not available on other rare items.