Electrons and ions generated in the liquid argon outside the field cage can drift until they are intercepted by an insulator, which thereby becomes charged. This effect is something like charging of insulators due to corona discharge in high-voltage systems in air. Mitigation of this effect should be based on standard techniques in the high-voltage industry: use of insulators with rounded, not sharp corners; and in extreme high fields, surrounding the insulators with conductive "grading rings". https://en.wikipedia.org/wiki/Corona_ring#Grading_rings
The tech note reviews analytic calculations that support this lore.
This note is also available at the public site http://physics.princeton.edu/~mcdonald/examples/insulator.pdf