I got an answer from reddit.
"Because you're mining on a variable difficulty pool... Which means you start out mining super low diff, and then it auto adjusts to a different difficulty level so that you get X amount of shares per X amount of time.
And now for the obligatory mention of NO, if you restart the miner a million times you aren't making $$$$$$$$$$$. Low diff shares worth less than high diff ones." - CrateMayne