The worst bears always refer to old knowledge

Your trading strategy is not telling you to get out. But you hear knowledgeable experts (normally older economists with a lot of old knowledge) declaring that the market is going to go down a lot. "IT-stocks are overvalued, they are going to half soon, they are not worth this much".

Or you get to read too many of the bearish papers: "If the index does not make 11408 in the next two days we are going to see a fast down move". With too much knowledge of that type, your decision might be to get out earlier than your system tells you to (studying the charts you are able to see that it will signal a sell if only...). So you get out, and with a good profit which always feels like a good consolidating thought. This is making it even worse, you are doing wrong, but can show "proof" that it was a good idea.

If you let your fear control you, your profit will diminish

The stock might not go down as the experts says it should. It might continue to go up, and you are at the sidelines watching it go up. You comfort yourself with the profit you have, and probably with the knowledge that "it is not worth that much, it should really go down and not up".

Your knowledge has once again interfered with your strategy, your second guessing has once again destroyed the statistics behind the working trading system, and you are missing some of the profit you should have received if you traded according to your strategy. 

Since your system is based on the fact that it gets a number of big winners, it does not return the profit it should. If this happens too often, it might not even stay profitable, specially if you do second-guessing also when the stock goes down and you "know" that it is going to turn up soon.