Interesting, because your code ...
<<set $parentsSeparatedBecause to [
"dad abandoned <<mom>>",
"<<mom>> left dad",
"dad died before you were born"]>>
... shouldn't display anything.
What you likely want is to print (one element of) your array evaluating it. This is typically done with the "<<= ....>>" macro. Compare the two outputs here:
<<set $pc = {nationality: "Barsoonian"}>>
<<set $parentsSeparatedBecause to [
"dad abandoned <<mom>>",
"<<mom>> left dad"]>>
<<- $parentsSeparatedBecause.random()>> - <<= $parentsSeparatedBecause.random()>>