I also advise that you do your very best to put the jars away somewhere and try to forget about them for a month during the incubation period. I will admit I still check my jars once a week or so.

I would say that the main thing is that the cake shrinks a bit as it is colonized and therefore when you flip them the top vermiculite layer no longer fills the jar up to the brim. It may roll to one end of the jar (shift) exposing bare substrate to the top inside of the jar. during colonization, If the lids are on loose and there is a hole in the lid for gas exchange, then you might introduce contaminated air when you flip. 2 possible workarounds:

1) flip them immediately after innoculation.
(my method and i have found that jars colonize quicker this way {i do not feel elevated co2 will enhance colonization})
2) use filter discs between the jars and lid
so if the vermiculite top layer is compromised, you'll still have a protective barrier.

hope this helps