Many factors can cause your electric bills to vary. Most commonly they include weather and changes of habits or lifestyles.
In hotter weather there can be increased usage due to air conditioning or even window fans. Hotter weather also leads to higher humidity so some dehumidifiers may run more often and for long periods of time.
In colder weather electric consumption can be increased by use of central heating equipment or space heaters. Typically during the colder parts of the year it gets dark earlier and people stay inside more. This leads to more use of lights, televisions, radios and other electronic equipment.
Changes in habits or lifestyles can also lead to varying electric bills. During the holidays your family may cook more. You may have overnight guests. Cooking and guests can lead to more use of water (and hot water). If you have a well and/or an electric water heater this will lead to more electric usage.
Another major change may be the addition of a new family member. Bringing a baby into a home usually adds to more laundry being washed, higher desired room temperatures and more lighting at night.