Volume too sometimes jumps to 90% when I connect my headphones to my iPhone
iPhones remember the volume settings for different sound outputs. ie. if you have the iPhone speakers, and a set of plugged in headphones, and a set of bluetooth headphones, and a wireless speaker the phone will remember the volume of each of them. When you connect the relevant device the volume is set to where you left it.

In this case you're connecting your headphones and it's setting back to where you left it.

In addition, Sony have a Headphones Connect app that can you can use to set presets for different scenarios. Do you have that app configured?