Cracks in wood ...come from the wood relieving stress...( drying causing expansion and contraction, pressure expansion and contracting, could be heat expansion, etc../ depends on where you had this rack .....like in a pickup - where inside the vehicle the temp swings 50 degrees or more between day and night is a really bad idea - especially if its humid or real dry ....!!)....but you need to have someone evaluate what is causing this / and determine if its worth figuring out...
Fixing the crack is one thing ....maybe all it needs is a high quality wood glue (not elmers), maybe it needs some type of a mechanical fastener to make it tight ...??? / ...but the real craftsmanship comes in hiding the repair ( ideally by creating dust from the piece itself when its refinished...and then taking that dust and mixing it with just the right amout of glue - to create a custom wood filler, so it will accept the stain the same as the surrounding wood ). Now if you're really lucky - when its taken apart ....the crack will "dry fit" together...and disappear ( but that's like hitting the mega lotto if it happens that way )...
With even the best woodworkers, stock fitters, cabinet makers ....making an invisible repair depends on how visible the crack is now, where it occurs in the wood piece, etc....and even then, its probably a 50-50 proposition.
All of which costs money ....and it may not be worth doing ...vs just fixing it to make it stronger ( and a good high quality wood glue will do that - like TiteBond ) - if you can figure out what is causing the stress.
But we can't diagnose it over the internet either...find a good woodworker or stock maker to evaluate it.../ somebody who can put it under good light and look at it closely in person / and can discuss what happened while it was in this rack ...( temp range, humidity, etc).