This is likely due to a weight and balance issue. There is a definitive line for Y fwd and Y aft. Keep in mind that in real application this is probably not a safety issue as there is probably a 30% fudge factor built in. On a full flight it's more a takeoff weight issue, not weight and balance, but on a light load it can be a factor. It gets more critical the smaller the A/C. Without the regs and the fudge factor, an out of balance situation could occur with disastrous results.

Seem to remember a cargo DC-10 a long time ago that was out of balance and it was pitched so far back on takeoff that the pilot couldn't push the nose down resulting in a stall and a crash.

Put simply, the entire A/C balances like a teeter totter with the hinge point being at the wings. The horizontal stabilizer can make up for excess weight in the front or back, but only up to a point. That's why there is weight and balance.