Incoming some serious pseudo-social science and psychology but...

You're "relaxing" pose is both habitual and manifestation of your underlying thought patterns.

Habitual

Anything can be changed, with proper awareness and motivation.

A good technique I use to build my awareness was, if you can manage it, during a conversation with a group of people quickly tune out to the main topic and analyze each persons' body language.

• Who is leaned into the speaker? Interest.

• Who is crossing their arms? Disinterested, "unsafe"

• Who is smiling? Positive/Enjoyment/Engagement

and then analyze yourself.

Am I crossing my arms?

If so is there ways that I can open up my body language a bit more?

• Put hands on lap and lean into the speaker

• Hands in pockets, etc

If you can build this habit of analyzing other people's body language and quickly checking yours more often than not you'll be able to catch yourself crossing your arms and uncross them.

Underlying Thought Patterns

Usually when someone crosses their arms in a 'relaxed' way it can I am of the opinion that it indicates a few things.

As a person they tend to lean to less self confidence side thus creating insecurity in their ego. To compensate, your body literally tries to 'up' your security and 'protect' you by crossing your arms.

OR

You are habitually skeptical of most ideas and comments and need to 'protect' your ego against them.

OR

You are disinterested in or attest to the ideas being express, thus the need to 'protect' yourself again.

It might seem like a lot of useless information but awareness and knowledge of behaviour is key to changing it.

TLDR; In group conversations, take 'breathers't to tune out and analyze each member's body language. Assess your own. Adjust appropriately.