Food and drink choices cause natural fluctuations of blood glucose concentrations. Sugars and rapidly digestible carbohydrates cause peaks in blood glucose concentrations which are managed and returned to resting concentrations by insulin which the body produces/releases in response. Alcohol consumption can lead to low blood glucose - especially in diabetics.
It is healthier to avoid regular peaks and troughs of blood glucose concentration and eat more sustained release dietary carbohydrates as glucose sources. The ‘reactive (postprandial) hypoglycaemia’ condition in some individuals occurs within four-hours of eating when blood glucose concentrations fall - due often to (excess) insulin release in response to dietary digestible carbohydrate (over) loading.
Fasting and weight control procedures/regimes can lead to low blood glucose concentrations too - including after bariatric surgery.