If you assume that total pilot pay for the airline is fixed and pay goes linearly with seniority (just to keep it simple and concrete), then increasing the wages of entry-level pilots requires reducing the wages of senior pilots. If you further assume that all pilots are perfectly selfish (that is, they'll never vote themselves a pay cut), then 50% of the votes automatically go to keeping new pilot wages low.
There will also be a contingent of just-below-median pilots who have paid their dues and are looking forward to the fat paycheck that comes with being in the top 50%. Transferring pay from high-seniority to low-seniority hurts the lifetime wages of this group, so they'll vote against it, too.
Thus new pilots are paid a pittance and old pilots are paid handsomely. I'd think this still works even if the pay scale is strangely-shaped.