It could actually be a usability improvement.
It's possible people were misunderstanding the effect of the images button (prior to the changes it had no label and confusingly defaulted to ON) and thus re-using the form to correct their mistake once they realised all their images had vanished. Note the change is only 400 submissions of the form not 400 less users in beta.
In the month of October, 2005 people turned images off at the same time as turning beta on [1] In September, 2424 people turned images off at the same time as turning beta on [2]
There were also 188, 741 saves to Special:MobileOptions in October [3] compared to 195, 980 in November so it could also be some kind of seasonal dip.
I wouldn't panic fix but it would be interesting to run some user testing around it.
[1] select count(*) from MobileOptionsTracking_8101982 where event_beta = 'on' and event_images = 'off' and timestamp > 20141001000000 and timestamp < 20141101000000 [2] select count(*) from MobileOptionsTracking_8101982 where event_beta = 'on' and event_images = 'off' and timestamp > 20140901000000 and timestamp < 20141001000000 [3] select count(*) from MobileOptionsTracking_8101982 where timestamp
20141001000000 and timestamp < 20141101000000
[4] select count(*) from MobileOptionsTracking_8101982 where timestamp
20140901000000 and timestamp < 20141001000000