Battery voltage drops occur when the internal resistance of the battery is too high.
The value of the internal resistance depend on several factors :
- The temperature (the internal resistance is higher in colder weather)
- The number of cycles (In other words the number of discharges and charges)
- The age of the battery
- The balance between cells voltage
It's also very important to store your batteries at the right voltage. You can see on the attached article, a document that shows how the storing voltage affects the battery capacity.