My choir has a consistent tendency -- absent a metrenome or vigorous conducting -- to slow down for quieter dynamics (and then never regain the original pace). We have several individuals with good senses of pace; this is a collective problem. Why does this happen and how can we fix it? We don't want the director waving her arms that much on stage; we want to fix the root problem and perform with minimal conducting.
@Matthew Read gave some good suggestions, to which I'll add:
- Try rehearsing the problem sections at a wide variety of tempos, particularly ridiculously fast (once the choir knows the section reasonably well). Unwanted tempo changes become habitual. One way to break that habit is to go much faster (or slower, if the problem is acceleration) than desired, at a variety of tempos, before going back to the tempo you actually want.
- @Matthew Read alluded to this, but I'll expand on it. Anytime you need the ensemble members to pay more attention to your conducting, always conduct smaller, not larger. By conducting too large, the conductor is helping the ensemble slow down regardless of the tempo being conducted. Keep the pattern small and simple. In rehearsal, if you really believe the ensemble is not watching, keep making the pattern smaller and smaller, and eventually just stop conducting and see how long it takes the ensemble to notice. After this happens a couple times, even younger students are embarrassed enough to pay more attention (usually).
- Another rehearsal technique, so long as the ensemble understands it is not for performance, is to have the ensemble gently "sway" with the beat and try to keep the time moving. The physical motion of following the beat in this way is likely to call more attention to the fact that the tempo is slowing. Of course, rehearse without the swaying after a couple tries so that it does not become habit.
I can't speak to the psychological reasons or addressing them, but there are a couple things you can try that basically apply to all types of music.
1) Have the weaker members listen to and follow the stronger ones. Ensure they can hear them, of course; don't put them on opposite sides of the stage. The mediocre members will probably do well enough if the bad influence is removed.
2) The director doesn't need to flail in order to indicate the tempo. Small hand movements should be enough.
3) Not everyone can keep a consistent beat, but everyone can follow a beat. If choir members refuse to do the latter by following others or the director, they should be dismissed. (I'm assuming your choir is "serious". If not, it may be something you just have to deal with.)