Apparently it happened on arrival from what I read elsewhere. The standard is to have bags start to be scanned 2 minutes after arrival. So ground crew is rushed to get the baggage doors open.
Also this aircraft had an inoperable APU (auxiliary power unit). Aircraft have essentially 3 power sources: the engines, the terminal when on the ground, and the APU. It takes time to hook up to ground power, and normally they cut the engines and use the APU while waiting to be hooked up to the terminal for power. But for this aircraft, they had to keep the engines going. Sometimes the pilots can shut if one engine, but apparently did not here. 
Likely in the rush to get to the baggage door, they didn’t take note of the engine and approached too soon. Also, the airport mostly sees smaller jets with tail mounted engines. The Embraer 175 was one of the few wing mounted engines they would see, which are far more dangerous to people on the ground.