Sometimes it happens, sometimes it doesn't.
try changing the size of the scrunchies (or whatever you are using to hold the sheets doubled) so that they edges don't touch (this is difficult but not impossible)
the inherent delicacy of the emulsion layers and the vigour of agitation will also be key components. you can control the latter but not the former.
if it's essential to you that even the outermost edges of the film remain perfect, then you might need to change your development method - perhaps buy a MOD54, or perhaps develop your films one at a time in trays (although the latter would be a proper challenge as you appear to be using colour negative films)