Each grammatical construction serves a function, such as conveying that part an utterance is at-issue or is backgrounded. When multiple constructions combine to produce an utterance, their functions must be compatible. This preregistered study (N = 680) addresses the enigmatic case of “syntactic island constraints”: Long-distance dependency constructions (LDDs) do not combine equally well with all base constructions. While widely presumed to require unlearned syntactic constraints, we test the idea that it is infelicitous to make an element both prominent (via an LDD construction) and backgrounded (via the base construction). Using 10 base constructions of English (144 base stimuli), results confirm two independent measures of backgroundedness strongly correlate with acceptability ratings on each of three LDD constructions. Results indicate that “island” constraints arise from a clash between the functions of the constructions being combined.