Strictly cowardice. One hundred percent. Not one of the companies listed below is acting on some sort of moral objection to the NRA and its positions on gun rights. Those positions have long been known by all of them, and they have not changed since Parkland.
What has changed is that activists on Twitter have started threatening corporations with boycotts unless they disown the NRA in any and every way conceivable. So a perfectly legitimate organization that negotiated group discounts with major companies will not see those companies welch on those agreements, for no other reason than that they are terrified of the criticism they'll get on social media if they don't: