Ampersands and accidental backgrounding aside, it's also possible that system encountered a problem but since you have no error checking on the call it's blithely continuing on to the next iteration immediately with no indication a failure occured.

$ perl -le 'for (0..3) { system( "Idonotexist $_" ) }'
$ perl -le 'for (0..3) { system( "Idonotexist $_" ) == 0 or warn "syst
+em problem (exit $?): $!\n" }'
system problem (exit -1): No such file or directory
system problem (exit -1): No such file or directory
system problem (exit -1): No such file or directory
system problem (exit -1): No such file or directory
$
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Update: Not to say that's what happened here since as you say you did see a lot of processes running; just that in general it pays to take the extra 15 seconds and put the error handling in place.