Food consumption
When we consume foods high in sugars or carbs, our bodies react by turning it into glucose and other energy molecules. This causes a sudden rise in your blood glucose levels.
Intense exercise
During intense workouts, our bodies release glucose as a response to the stressor (exercise in this case). This is actually different from a food-related rise and is not a "bad" spike. During particularly intense workouts, the stress hormones in our bodies tell the liver to release stored glycogen in order to fuel the "fight or flight" response that is happening.
Stress
Stress causes an increase in cortisol, and may cause our bodies to release extra glucose, which can cause a spike.
Example of a post-meal spike
These are not the only causes of a glucose spike, but are the most common ones. For example, a cold shower can also result in a glucose rise sometimes, due to the sudden shock we give to our body when we step into a cold shower. A sudden and extreme change in temperature prompts our brain to put the body into an ‘emergency warm up’ mode to ensure your core temperature doesn’t drop too low.