There is a scene in Avengers: Endgame where Cap and Thor call for their weapons and then Cap ended up with Stormbreaker and Thor ended up with Mjölnir. Then they swapped.
I can make sense on why Cap CAN hold Stormbreaker but why did the weapons go to the other person? Like were the weapons confused who they should be going to or something?
Is there an explanation for this?
That much is obvious. In this situation, it was mainly just a way to break up the action, Thor demanding that he gets the bigger weapon.
That said, the origins of Mjolnir, and Stormbreaker, explain that they work in similar ways:
Mjolnir itself has several enchantments: […] it returns to the exact spot from which it is thrown and returns to its Thor when summoned
Beta Ray Bill's weapon, Stormbreaker, also has the same properties as Thor's hammer, Mjolnir
That said, Beta Ray Bill obviously does not (directly) exist in the Marvel Cinematic Universe, so Stormbreaker is not a link to him in any way. The process of creating Stormbreaker in Infinity War was simply a way to replace Mjolnir.
"I'm gonna hold it open."
"That's suicide."
"So is facing Thanos without that axe."
Over the years, "summoning" Mjolnir (and by extension, Stormbreaker) has become ambiguated into "Thor calls his hammer", with little more than that for explanation. There is no real explanation as to how he calls it, or what he thinks, or feels when it happens. So it's anyone's guess. My only suggestion is that it might be that both Cap and Thor call the weapons, and based on their proximity, and the angle of approach that the weapons have, it'd probably work smoother to have them arrive at the wrong person, rather than bump into each other on arrival (though that would probably be arguably just as funny).
TLDR: Mjolnir gave Cap the ability to summon Uru weapons.
My theory: When both Thor and Cap called for the weapons in that scene, they were calling the ones they received. It's easily possible that in the heat of the moment Cap was aware of Stormbreaker's location and Thor was aware of Mjolnir's location, but not vise versa. That gives the chance to do the weapon swap joke. Now, the real question is how is Cap able to even summon Stormbreaker. He can summon Mjolnir because he is worthy, but Stormbreaker doesn't have the worthiness enchantment on it. So, that brings up the question, how does Thor summon Stormbreaker without the enchantment and how did he summon Mjolnir before Odin put the enchantment on it? I think all weapons made from Uru by the Asgardian's personal blacksmiths automatically come with the ability to be summoned by anyone in the Allfather bloodline. Notice how Odin could summon it away from Thor. Might also play a part in why Hela could catch it so easily after just coming back from imprisonment and nowhere near full power. When Cap weilded Mjolnir he possessed the power of Thor. One of those powers is the ability to summon royal Asgardian weapons. Cap wasn't holding Mjolnir when he summoned Stormbreaker and the enchantment does specifically say they have the power of Thor when holding the hammer. I think they stretched it for the MCU and made it so they still have the power of Thor for a short while after wielding Mjolnir.
I know it's been a couple months since this got answered, but I have a new theory that, after looking at the Endgame scene again, I think is confirmed.
We know that they don't have the exact same enchantment as only those worthy of Mjolnir can wield it. And we also know that the enchantment doesn't exist on Stormbreaker as even Thanos is able to hold it and almost kill Thor with it.
However, actively calling it seems to be a different scenario than actually wielding it. I'd be willing to bet that you must be worthy to actually call both weapons, (Thanos/everyone else can't call Stormbreaker) but you don't need to be worthy to use both weapons, only Mjolnir.
Now how does this answer the question? Well if both weapons have the same 'call' enchantment on them and if both Thor and Cap call a weapon (doesn't matter which one), it'll first look in the general direction of where it's being called from. So Cap called a "Thor-worthy" weapon and happened to get Stormbreaker (which was in the direction Thor just threw it) and Thor did the same in the opposite direction.
You'll also notice in the scene as soon as he calls Stormbreaker, once it's in his hand, he does a double take (and so does Thor). Both are visibly confused at what just happened. They both called weapons but they didn't call weapons specifically.
You can see it more clearly in this time-stamped video
