There are many points which don't make sense to me.
The biggest reason why some have been wiped out I will put it out to the central bank expectations which are dictating the markets across asset classes, not only FX. That's by far the most important thing going over this summer - the getting out of the massive printing room.
But if you disregard that, though you MUST NOT, then:
- "Summer the trend tend to do one way ,slow but long ." ---> If you capture a trend, just go with the trend. Be patient. Why not follow a trend if you actually identified there is a trend? Speaking of the USD, some big fishes must have been following the USD/JPY long trade for over 6-7 months now, if you look at public long/short contracts.
- "It is very dangerous and hard to make profit if market slow and less violated ." ---> You mean volatile. Yes, lesser volatility means lesser profits but the opposite is true too. Less volatility means lesser chance of losing more and losing it fast. So that holds against people getting wiped out. But again I disagree. There were several CB driven events that pushed up volatility pretty high over the summer. Check for that the USD/JPY's latest vol going up quite a bit.