Why do olympic runners slow down right before the finish line?
The main reason that Olympic runners slow down right before the finish line is that they're in a preliminary heat, so they only have to be in the top 2 or 3 places to move on to the next round. They'll often slow down when they're leading by a comfortable margin and don't want to burn themselves out for a later race. It's also possible that some of them do it to avoid injuries, since that's always a concern. But overall it's much less likely that a runner will slow down near the finish line in a gold medal race.
Occasionally, a runner will be so far ahead that they will slow down a little bit just to make sure they finish the race with no problems, such as tripping or twisting an ankle. This is much more likely in longer races. Some may slow down a little as part of a victory celebration. In other cases, fatigue and/or the build up of lactic acid affects them and there's no way they can avoid slowing down.
Athletes have been known to make the mistake of slowing down too early, thinking that they have a big enough lead and then being beaten out by other runners.