Hi. A minus power is called myopia. While seen commonly in the population, anything beyond -6.0 is termed pathological myopia.
There are various reasons for high minus power. Genetics do play a role ( if either parents have high minus power, children may get it)
Certain actions can exaggerate minus power-
Excessive use of near vision stimulation (especially in children)
Exposure to sunlight is known empirically to reduce rate of progression.
Use of low concentration of a drug called Atropine eye drops has shown in clinical studies to reduce rate of progression, or however does not reduce the eye number.
Next Steps
Your options are to take steps to reduce further progression.
once power stabilises, you can opt for surgical reduction in eye power (if you're open to surgery)
Health Tips
Generally eye power stabilises around 18-22 give or take a few years.