While I was at the convention this year, I don't know that I have much insight into the matter. This is complicated in a parliamentary way but not so much on the core issue. Baptist Press reported it .
The SBC has an app for it's annual meetings which includes a button for "social media." I'm not much for Twitter but that button on the app gave users an unfiltered stream of tweets and before the convention the stream was dominated by vile racist white supremacists. This is what many younger and minority SBCers saw as the alt-right and were aghast that the SBC Resolutions Committee did not report out the previously submitted resolution against it. My conjecture is that older, white SBCers may have seen the label "alt-right" as including a broader segment of political conservatives and Trump supporters. The RC said as much ( without mentioning Trump, along with a couple of other reasons) in explaining why they didn't report the original resolution out to the floor.
Once the committee made that decision there was a parliamentary hurdle, a two-thirds vote, to overcome to get it to the floor of the convention. Twice votes were taken to do this and it failed twice to reach the required super-majority, not because any of the 5,000 or so messengers favored the alt-right but rather because most accepted the explanation of the Resolutions Committee that the resolution submitted was unnecessary, overly broad, and poorly worded. Once the committee and other leaders were convinced that the convention was headed for a train wreck on this they quickly moved to pass the matter. The RC chair was unusually candid in apologizing for the mess.
Resolutions at the SBC annual meeting are just opinions but are important in the sense that they are watched, as much as anything else that happens at the yearly confab, by the secular media and public. The main job of the Resolutions Committee is to be sure we don't screw up with them. They almost failed on this one.