What is the deal with the Summer Lull?

It seems that the beginning of summer Bitcoin performs rather bad, but in the end of summer it tend to get more upward action.

2022 June -> July ?% July -> August ?% (Bearmarket)
2021 June -> July -20% July -> August 47%
2020 June -> July -2% July -> August 29%
2019 June -> July 22% July -> August -7%
2018 June -> July -1% July -> August -2% (Bearmarket)
2017 June -> July -18% July -> August 111%
2016 June -> July -3% July -> August -14%
2015 June -> July 20% July -> August -8% (Bearmarket)

Comment:

This is from 15 June - 15 July and 15 July -> 15 August