48-year-old Rider -- who spent 9 seasons in the NBA -- is the head coach of the SkyRiders basketball team, which was playing a tourney at the Inspire Court gym in Mesa, AZ on May 17.

Cops responded to the tournament over reports of parents fighting -- and when they got there, they spoke with Isaiah's wife, Vanessa ... who said she had been attacked by another woman for "no viable reason."

Vanessa Rider also claimed her 9-year-old son was physically assaulted by an older boy and injured during the altercation.

When cops arrived, they found the other woman ... but she told a completely different story.

The woman claims her son's team had defeated the SkyRiders so Vanessa was bitter -- with Vanessa calling her a "bitch" and trying to swing at her.

The woman claims she fought back to "protect herself" and admits she connected with Vanessa.

That's when the woman claims Isaiah got involved -- jumping over bleachers and calling for his friends to help break up the situation.

Isaiah allegedly started asking, "Who hit my son?" -- and then "told his friends to get his gun out of his car."

We spoke with Vanessa who is adamant that someone else in the gym had threatened to pull a gun first -- and Isaiah reacted in an effort to protect his family.

The woman claims Isaiah left the building but never came back with his gun. Vanessa confirms to TMZ Sports her husband NEVER actually pulled a weapon.

Cops ultimately determined the women were involved in "mutual combat" -- but since they couldn't determine who started the fight, no one was arrested.