It's because changes in Testosterone levels cause acne.
Sebaceous glands produce skin oils and waxes (sebum). Sebum collects inside sebacytes(specialized cells) within your pores. As new cells are produced, they push existing oil filled sebacytes to the surface of your skin(called "turnover") where they break open and spill their contents, moisturizing skin.
A sudden increase in Testosterone stimulates sebaceous glands to produce more sebum, but cell turnover doesn't immediately speed up, so pores become clogged with oil filed cells, they break open and decompose within the pore, causing an infection.
When Testosterone levels drop, cell turnover slows before oil production does, so the same problem happens.
The way to minimize this is to keep things stable. Eventually cell turnover speeds up enough to match increased oil production.
Knowing you're prone to this, whenever doing anything that can impact your hormone levels, be extra diligent cleansing your skin with salicylic acid body wash for the first couple of weeks. If it's really bad, 100mg doxycycline taken daily for a couple weeks at the same time you make the change can prevent it.