Bunnies like to explore their surroundings and will spend time around the edges of your yard where they feel safe. It's natural for them to shape their habitat by digging and chewing and can quickly open up a space under a fence or through a hedge big enough to squeeze through.
Even if your fence goes down to the ground bunnies can quickly excavate a hole big enough for them go squeeze through.
A sure way to stop this is to bury something under the fence like a line of bricks or another piece of wooden fencing.
A great way to stop your bunny digging is simply to lay a paving stone over the spot where you want to stop it.
If you have a hedge, then the best way to make it escape proof is to use some wire mesh. Make sure you bury it a bit into the ground so it can't be undermined.
Back gates nearly always need some improvement and it especially important to bunny proof them if they lead to dangers outside such as a busy street or areas where people walk their dogs. Extend the gate so there is no space below the bottom of the gate and the ground.
Gates with self-closing mechanisms are ideas to help you get in and out easily and with other family members and guests who may not be initiated in stopping bunnies escaping.
Remember the edge of your yard will constantly be tested by your bunny so make sure you check regularly to see if an escape route is developing under a hedge or behind some furniture.