Many ancient civilizations flourished once upon a time. Some slowly died out. Some disappeared overnight. A singular cause or varied?

There have been many reasons.

Natural causes such as diseases and long-term droughts, man-made ones like external and civil wars or general decay... The Roman Empire faded away largely due to its own degeneration, but it was also becoming increasingly difficult to maintain its far-flung empire against invaders. 

What is likely to be our civilisation's demise - looking beyond the USA's febrile internal politics which only affect the USA anyway?

Well, the SARS-Covid-19 virus and its offspring are having a good go at it, and do show what pandemics can be capable of; but I think the far greater threats to ourselves will be are ourselves, by:-

 - the major effects of climate-change,
 - increasingly rapid depletion of natural resources with no satisfactory long-term replacements,

with those two answered by knee-jerk reactions that do not seem based on rational thinking that also looks more than a couple of decades or so ahead;

 - greed and intolerance between different nations and cultures,
 - poverty and injustice,
 - and the one the politicians dare not mention, or do so only very quietly, but is implicit in all those above: human population.