The peak is determined mathematically and statistically.  Mathematics (and physics) is the science of the nature.

 Say that you have some goods that you want to sell. Example plumes. When it is summer you have many plumes and you sell them for a lower price. When winter comes, you have less plumes and you increase the price. You can not increase the price too much because people will not buy from you.

This is what happens in nature.  There is an average and a deviation of the prices (say) from this average. 97.8% of the events happen in 2 standard deviations from the average.  

When the bitcoin price has increased (gone up, appreciated) in 2017, it had gone to approx 3 standard deviations from the average value.  In 3 standard deviations 99.9% of the events in nature happen.

When the bitcoin price will start to increase (to go up, to appreciate) very probably in the fall of 2019 the peak will be (probably in 2023) approx 3.1 standard deviations.

Prices start to increase in the fall, because winter comes.