A tragic incident.
Unknown what actually happened, just based upon the article.
While the cause of this tragic accident may not be determined (or released), there are some situations that can happen which can increase the potential for unintended discharge while handling a gun being holstered or withdrawn from a holster.
Having an errant index finger getting caught inside the trigger guard and shoved against a trigger, by the holster mouth, when holstering.
Getting clothing caught up inside the trigger guard when holstering (especially with lighter triggers which may also include a trigger safety that's disabled when pressure is applied against the trigger face).
Having a gun released and dropped, and then trying to catch the gun, only to have one of the grasping fingers "find" the trigger, and all the fingers clench to control the gun ... unintentionally pulling the trigger.
Mishandling the gun (finger on trigger) and not paying attention to what's being covered by the muzzle.
From the equipment perspective:
A damaged/defective gun mechanism, or improperly maintained (or modified, etc) gun that develops a safety problem.
A holster not suitable for the specific model gun being carried ... or with a defect ... or not properly maintained, and which develops a defect or other problem ... or one with which the owner/user is unfamiliar with proper usage ... or one which might not be suitable or most appropriate for use under the conditions planned for its use.
Want to see something scary? Wait until the first few times you see another shooter have some part of their clothing (like a draw string, or the leading edge of a lightweight jacket) get caught up inside the trigger guard as they shove a gun back into a holster.
Or, see them struggling to shove a gun into the holster, not realizing the safety retention strap has become entangled inside the trigger guard
in front of the trigger face.
Holstering is something that requires not only requires the proper equipment choice, but also some serious attention to learning a safe & effective holstering technique, appropriate to the holster design.
It's NOT an afterthought, or something to be rushed through as if it's not an important aspect of safe gun handling.
Speaking about holster maintenance and wear, remember this one?
SAFETY WARNING! Worn Leather Holsters Can Cause Accidental Discharges! | Gunlife Resource Center