Really sudden plant death can be caused by gas - either a leak from a man-made gas supply or a natural emission (mainly in volcanic terrain). Such a gas leak would probably also kill adjacent plants. If only the plum tree is affected, and there are other plants adjacent, gas won't be the cause.
But perceived sudden death in woody plants can also be the last stage of a longer-established problem. A canker, for instance, can gradually work its way round the trunk, and the tree can get-by surprisingly well until the canker completes the girdling. Initially, that will stop the flow of sap to the roots, but then the roots stop working and the supply to the leaves (and any fruit) ceases. Various fungi and other pathogens can cause similar effects, with relatively sudden decline and death eg. Armillaria (honey fungus), and Phytophthora.
Plums are susceptible to more diseases than most trees (a feeling; I don't have data to back that up). Silver leaf (Chondrostereum purpureum), for instance, goes for Prunus (but that's unlikely to be the agent in your case, as it usually kills a tree a branch at a time).
It would be worthwhile prising up bark at soil level and excavating the roots a bit, looking for signs of fungal bodies. If some are evident, they may be the causative agent. They could also be secondary, but it might be a bit soon for secondary infections to establish and get to a recognizable fruiting stage. It is important to check for fungal pathogens, as they could spread to other trees or shrubs.
Some refs:
-
http://www.petercoppin.com/factsheets/pests/plantdie.pdf-
http://www.cotswoldtreesurgeons.co.uk/sudden-tree-death-
https://ask.extension.org/questions/305929Mike