Not because some war was imminent, but because of very practical reasons: Belgian police could only reach that piece of Belgium by boat; so "exchanging it with NL" was the easiest, because the Dutch police could just drive there.
It makes me happy to see that we're down to "practical exchanges of land" from centuries of war over the most silly pole, church or "slight".
[1] https://nos.nl/artikel/2112869-nederland-krijgt-belgisch-sch... Dutch only, sorry.