propaganda.
2 simple reasons,
the first is that you may simply be going off too fast and burning out too quickly, dropping your pce significantly to compenstate giving you some recovery to push the finish.
the second is that you are just losing focus during the race. This is all about race tactics. throughout the race I would be checking on my pace, is it enough, is it too much - fine tune as you go along. Don't get stuck behind a slower runner and don't try to chase down a faster runner. If there is another runner who is keeping good pace then try and keep up but not to the detriment of your own race.
Small races are worse for me, if I lose sight of the runners in front then I quickly lose focus. It maybe worth using a stop watch to check your progress though I just find this a distraction unless I have times scrawled on my arm so I don't have to do the maths.
As for training, are you doing tempo runs? I find these best for getting the pace right. Are you getting the miles in throughout the week? Are your long runs long enough and at the right pace? Are you giving yourself time off during the week prior to your races.
There really is so much fine tuning that you can do. I'm no expert. Are you actually targetting a race and training for it or just going out and racing without specific training?