There were more relocations that preceded it, but the move of the Charlotte Hornets to New Orleans is arguably the first domino that led to the relocation of the Seattle SuperSonics to Oklahoma City.
The Charlotte Hornets and the Vancouver Grizzlies simultaneously applied for relocation to Memphis, Tennessee on March 26, 2001. This led then-Hornets owner George Shinn to return to Charlotte with his hat in hand. He issued an ultimatum to the city, asking them to build him a brand new arena at no cost to himself or else he would move the team. The city refused, and Shinn began to look elsewhere. Eventually the city of Charlotte agreed to build a new arena, but only if Shinn sold the team. Shinn responded by taking his ball and going to New Orleans.
The second domino to fall happened on August 29, 2005, when the city of New Orleans was ravaged by Hurricane Katrina. The storm partially destroyed the New Orleans Arena, where the Hornets played their home games. Several cities inquired to the NBA about hosting the Hornets while the arena was repaired, including Kansas City, Louisville, Nashville and Oklahoma City. OKC would ultimately win the bid and the team played 35 games in the then-Ford Center during the 2005-06 season. They were referred to, officially, as the New Orleans/Oklahoma City Hornets. Fans in New Orleans became infuriated when the team placed "Oklahoma City" on their jerseys. They felt the league was disrespecting them by letting the babysitter rename the child.
Despite the arena being ready for the 2006-07 season, the NBA instead decided to keep the team in Oklahoma City so the market could "fully recover." In reality, it was a tryout for Oklahoma City, to see if they could maintain the fandom shown in 2005 for a second consecutive season. It turns out they could, with the team ranking in the top half of attendance for the two years it spent playing its home games away from home.
When the team moved back to the bayou, the attendance was once again in the cellar. Their first year back, they ranked 26th in attendance at 14,181 per game. They have not climbed higher than 19th since. The now-Pelicans are currently 21st at 16,350 per game. Meanwhile, the NBA had found a new market in Oklahoma City and it wanted to be there. The smart idea would have been to just keep the Hornets there. George Shinn wanted to sell. They were already there and building a fan base. It would have been very easy. Big Easy, one could say (no one should say that). However, the NBA didn't want to appear as a heartless, soulless company (LOL) who would turn their back on a city that was just destroyed by a terrible natural disaster. So the Hornets returned to New Orleans, and we know what happened with Oklahoma City getting a team all too well.
So... what if none of that ever happened? What if the Charlotte Hornets were still in North Carolina? Would the Sonics still be in Seattle? Or would they have moved to Kansas City or Louisville by now? Would we be talking about moving the Bucks to Oklahoma or even New Orleans? Would we be gearing up for the Playoffs with Kevin Durant and Russell Westbrook in green and gold? Possibly not, considering if the team hadn't been sold to Clay Bennett, then Sam Presti may never have been hired and we have drafted some 7-footer who'd be playing in China by now.
So many questions we will never know the answer to. Everything is changed by everything. Even something as simple as a Hornet flapping its wing can change... everything.