In this visual we can see an orange graph overlaid onto the BTC price chart for the last peak back in April. This orange graph represents the collective market cap for all crypto excluding BTC.
We can see that the price of altcoins lag behind the price of BTC and actually top around around 3 weeks after the peak. If you go back and look at the ATH at the end of 2017 you will see a very similar pattern.
Altcoins tend to lag a few weeks behind BTC and, therefore, peak after BTC highs also. If you are an investor, this is a key sign to watch out for and if you have made significant gains will give you a strategic exit.