Welcome to wood! Wood is a natural product and it "moves" - it's a feature that can cause failure. Without a picture, it's had to know if it's really "buckled", or if it's just "warped" a bit, but my money is on "warped".
To prevent collapse of the bed while in use, you could drill a hole in each end of the center slat and through into each side rail, then screw it together while in use to prevent the rails from spreading, then unscrew it to disassemble and store away. It doesn't even need to be screwed tightly, so if the screw threads chew up the wood through frequent insertion/removal, it doesn't matter too much. Their purpose isn't to hold the slat tightly to the rail, but to keep the rails from spreading. You could even use a wooden peg instead of a screw.
You could drill a hole for a wire and use a turnbuckle to keep them together, but this strikes me as overkill, and you'd need to put an eye-bolt (or something) through each side rail to connect your cable. This would look ugly and would be something for bed sheets or shins to get caught on, so I wouldn't recommend it.
A note on storage:
Since this seems to be something that you assemble and disassemble as needed, the way you store it when not in use could impact how quickly the wood warps.
If you lean long (even 1-2 feet) pieces of wood vertically against a wall, you will speed up the warping - they'll tend to bow toward the wall/floor intersection. If you lay them flat on the floor, they'll tend to stay straighter longer, but can still bow simply because it's a natural product and that's what it does. Since the longer sides have bowed, you might be able to straighten them a bit by storing it with the bow up and putting some additional weight (like the other side) on top of it.
There are other methods of straightening bowed wood, but I would suggest you head over to the woodworking sister site and peruse the questions there for additional tips. TBH, I doubt you'll need to go to any great extremes for quite some time with this bed frame.