What is the common pin configuration upon microcontroller reset?
From the app note by ST (AN3430):


  After a reset, all STM32F2 pins are configured as inputs except some JTAG pins such as PA15 JTDI with pull-up, PA14 JTCK with pull-down and PA13 JTMS with pull-up.


It makes sense to have high-Z state after reset because it is the safest default state. In case of STM32F2 series micros, it also has the least power consumption.