There are no currently known abiotic production routes in Venus's atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery.
It is either unknown photochemistry or geochemistry or there is some kind of life in the atmosphere of Venus.
The surface of Venus is deadly to life with a lot of acid, pressures and high temperature. At 55 km (34 mi) altitude on Venus, the atmosphere of Venus is 27 °C (81 °F) and 0.5 bar (50 kPa). 0.5 bar is half the pressure of Earth at sea level and is the equivalent pressure at an elevation of about 5,500 m (18,000 ft) on Earth.
Asteroids have been hitting the Earth for billions of years. So small bits of rock with life from Earth could easily travel to Venus and seed the upper atmosphere. Venus at 34 miles if very hospitable for life.
Some high altitude probe will need to be sent to Venus to determine if there is life or what processes are creating the Phosphine.