Node Async Looping - Why does the memory grow and just drop all of the sudden?

I'm writing a program in Node that uses an async loop. The goal is to get this program to run on Heroku for an extended period of time. It grows in memory, as expected. But then once the memory usage hits about 57MiB, it drops back down to 22MiB (where it started). What causes the memory usage to drop out of nowhere like that?

Here is my code, if it helps at all. database.read is just a simplification of http.request.

var http = require("http");
var util = require('util');

var fnstraj = require("./predictors/fnstraj.js");
var database = require("./library/database.js");

var COUNT = 0;



////////////////
// Queue Loop //
////////////////
var worker = function() { 
    setTimeout(function() {
        COUNT++;

        console.log("Worker Clock: " + COUNT + ".");
        console.log(util.inspect(process.memoryUsage()));

        database.read('/queue/', function( results, error ) {
            if ( typeof error !== "undefined" && error ) {
                process.nextTick( worker );
            } else {
                var queue = results;

                database.read('/flights/', function ( results, error ) {
                    if ( typeof error !== "undefined" && error ) {
                        process.nextTick( worker );
                    } else {
                        var flights = results;

                        if ( !flights.error && typeof queue.rows[0] !== "undefined" ) {             
                            for ( flight in flights.rows ) {
                                if ( flights.rows[flight].doc._id === queue.rows[0].doc._id ) {     
                                    var thisFlight = flights.rows[flight].doc;

                                    console.log("Flight " + thisFlight._id + " started");

                                    thisFlight.duration = fnstraj.vertPred(thisFlight.launch.altitude, thisFlight.balloon.burst, thisFlight.balloon.radius, thisFlight.balloon.lift);

                                    fnstraj.predict(thisFlight, function() {
                                        database.remove('/queue/' + thisFlight._id);

                                        console.log("Flight " + thisFlight._id + " completed");

                                        process.nextTick( worker );
                                    });

                                    var found = true;
                                }
                            }

                            if ( !found ) {
                                process.nextTick( worker );
                            }
                        }
                    }
                });
            }
        });
    }, 25);
};