I can confirm akebrakes' comment. If soft isolators are used close to their max load (= more efficient isolation down low in frequency), the speaker rocks back and forth like a ship in a storm if you push it gently. For my flush mounted speakers I made a very simple, cheap but effective device to stop any motion. Shown here: find out a good heigth for flush mounting speakers

If there is is place above a flushmounted too light speaker, the idea can be used to put additional load onto the speaker / isolators, instead of putting something heavy on top of it. Check the manufacturers' data on max compression of the isolator and measure with a caliper how much room there is left under the speaker as you screw down the Allen screws from above. (I imagine this may be somewhat similar to Northwards' way of adding enough load on the spring isolators under the speakers).

The installed speaker in the doghouse is shown in bit further up here: find out a good heigth for flush mounting speakers