It's strange, but being marooned wasn't as unusual as one might think.
William Dampier (who Selkirk served with some years before being marooned)[1] had himself been marooned in the Nicobar Islands:
Later that year, by agreement, Dampier and two shipmates were marooned on one of the Nicobar Islands.[2]
I believe this was mostly about disagreements at sea with commanders. Many empty islands had been stocked with animals such as goats and rabbits in case of shipwreck, and would have ships call at them a few times a year for water and resupply.
It does seem an odd thing to modern thinking though.
William Dampier (who Selkirk served with some years before being marooned)[1] had himself been marooned in the Nicobar Islands:
Later that year, by agreement, Dampier and two shipmates were marooned on one of the Nicobar Islands.[2]
I believe this was mostly about disagreements at sea with commanders. Many empty islands had been stocked with animals such as goats and rabbits in case of shipwreck, and would have ships call at them a few times a year for water and resupply.
It does seem an odd thing to modern thinking though.
[1] https://en.m.wikipedia.org/wiki/Alexander_Selkirk
[2] https://en.m.wikipedia.org/wiki/William_Dampier