Thor's arm loss in the comics can occur due to multiple reasons. Sometimes it's the result of a magical curse or a confrontation with an extremely powerful villain. The details change across different comic arcs.
Rick lost his arm when he faced a challenging situation. The circumstances were overwhelming, and despite his best efforts, he couldn't avoid the injury.
The way Oliver Queen loses his arm in comics isn't always the same. Sometimes it's a result of a risky mission gone wrong, or it could be from a confrontation with a villain with unique abilities. It all depends on the particular comic storyline.
Misty Knight loses her arm in a rather tragic incident. She gets caught in a dangerous confrontation where she underestimates the enemy's power and pays a heavy price.