Generally, was the workforce reduction the result of employer action - mass layoff, plant closure, sale of division, etc.? Usually any termination is deemed employer initiated, so employer should have clear detailed records in order to claim employee initiated voluntary terminations.
If most or all of terminated participants were replaced by new employees, then maybe this is a situation of normal turnover for this employer (industry probably plays a role here as well). If similar turnover has occurred before there was plan, that is further evidence supporting position that this wasn't a partial termination.
If 5500 filing shows a 20%+ reduction, employer should expect a letter of inquiry and have evidence to argue against partial termination.