When the schedule for this season originally came out, many probably saw this as a game in which the Spurs would get owned. Knowing they would likely be jockeying for lottery position by this point and presuming the Dallas Mavericks would be playing for postseason seeding, there was no reason not to see the season finale as a scheduled loss.
Instead, the disappointing Mavs entered freshly in tank mode to try and preserve what slim change they have of maintaining their first-round pick, sitting even more players than the Spurs, and the blowout may as well have been on before the game began.