Catching a flight with just a carry-on can make travel feel easy. No need to check a bag and all your belongings stay with you. But quite often, that suddenly changes before boarding.

Why do airlines make passengers gate-check their carry-on luggage? Turns out, it comes down to the size of the plane.

"This happens a lot, especially on shorter flights that are flying regional aircraft," said Gunnar Olson, travel expert and host of the Thrifty Traveler Podcast. "Usually, they're going to make you gate check your carry-on when the plane is just too small to handle the amount of carry-on bags that are onboard."

The reason gate checking bags is becoming more common is checked bag fees keep rising, said Olson. Rather than paying to check a bag, more passengers are packing everything into a carry-on.

"The planes are not built; they're not equipped to handle everybody bringing a full-sized bag onboard," he said.

Some passengers have reported seeing empty space in the overhead bins after getting their bags gate-checked. In that case, why force people to separate from their bags? The answer is time.

A spokesperson for Delta Airlines told CBS News Minnesota, "Boarding is the most consistent barrier to an on-time departure, and the boarding process can be slowed down by customers needing extra time to store their bags."

Delta uses "optimization tools" to predict if the overhead bins will be full during a flight. The airline said asking customers to check their bags at the gate is "always a last resort."

If a flight is expected to be full, Delta also gives customers the option to check their bags at the lobby kiosks for free. One traveler had a similar experience for a recent flight with a different airline.

"Canada Air warned me when I checked in here at the airport that it was gonna be a full flight and they recommended I just check my bag for free. So I did," she said.

How can you avoid having your carry-on gate-checked? It starts when you book your ticket.

"My only advice would be to try and buy up to the next higher fare that would get you on board a little bit earlier," said Olson.

By boarding earlier, he said, you raise your likelihood of getting access to overhead bin space.

His point rings true with Southwest Airlines. A spokesperson for the carrier told WCCO, "Almost always, we request bags only from customers in the final boarding groups. We reserve certain bins for customers who purchased extra legroom seats."

Delta and Southwest were among several airlines that raised their checked bag fees this spring. The spike was to help offset the rise in jet fuel costs from the war with Iran. The first checked bag for Delta and Southwest now costs $45, an increase of $10 from the previous price.

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