Okay, okay okay, we said it! Seasonal shedding is a myth!
Our lashes take about 60-90 days to go through the growth cycle and you typically shed every day about 1-5 natural lashes. While this is normal, seeing more shedding from time to time is going to occur. Changes in diet, exercise, etc can alter your growth.
For example, if you see a natural lash attached to the extension it was that lashes time to shed. If you see an extension without a natural lash on it then the bond was broken or weakened and it popped off from over-brushing, or attachment most likely.
You may notice your lashes growing faster/slower but it depends on your body's hormones. Summer weather can cause some clients' skin to sweat and produce more oils, and Winter weather with its lower humidity levels can cause lashes to become more brittle. Season does not change your growth rate, your body's chemical make up is what changes your growth.
When they say beauty is only skin deep, they aren't kidding. What you put into your body is what your body will put out. Maintain a healthy lifestyle inside and out to keep your lashes growing strong, and remember hormones can affect your lash growth. During pregnancy you will see excessive growth and shedding due to hormone levels, vitamins, etc.
Always consult a doctor for any concerns.