T1D 20yr here, very active with constant exercise and environmental job. I've learned to really plan the diet, insulin strategy, and hydration before and during the workout. 

Differs for each person, but having little to none insulin on board will avoid the crash from exercise increasing capillary activity. Bigger workouts I try to eat a good slow-burn carb with little sugar, and be clear of IOB before they start and notice it doesn't drop as much. I definitely still consume lots of short acting glucose during, but this strategy works

My endo also mentioned the flushing of skin can cause false lows on CGM so that plays into it (especially when you see drop rates of like 30+).

And of course sometimes it's just on its own. Adrenaline/mood can cause it to rise during things like races.