Price momentum sometimes reverses, sharply. The snapshot of the relative strength table below show a result of more than 1,000 ETFs ranked by their 6 month price change. Those in the image are the ETFs that had the lowest 6 month relative strength. For example, the worst 6 month momentum is down -49.91% the past 6 months. As you see, however, they have all have high 3 month price momentum. If a system applied a momentum ranking system that defines 'relative strength' as 6 month rate of change, it would probably have a materially different portfolio than if its parameter is 3 months. 

In the table below, we have again ranked the ETF universe by 6 month momentum, but this time I show the return for the highest 6 month relative strength. You may notice that the strength of their price trends hasn't been so strong more recently.

These observations are a normal element of relative strength ranking after a major trend change or high volatility. That is, about 6 months ago world markets were in a very volatile state and many markets were declining sharply as evidenced by the table. Since that time, some markets have reversed. As evidenced by these tables, some have reversed sharply. Whether or not a trend system hooks on to them depends on how adaptive the system is. 

Catching a big one.  (click for image source)

Don't get too aroused by the size of the price change - they magnify the variation between trend periods because they are mostly leveraged ETFs, which are far too volatile for most investors.