I had a patient come in yesterday with a great pair of 3-strap adjustable sandals. When I had her slip them on and walk, I was amazed at how poorly they stayed on her foot. Walking down the hall, her heel was slipping sideways and her arch appeared to drag nearly on the ground. Because none of us spend much time adjusting straps when we purchase sandals, the following is a brief instructional on a one time correct fit maneuver. Once done, you don’t have to adjust again. This simple adjustment sequence can mean the difference between comfort and sloppy.
If you have a sandal with one of the straps being behind the heel and the other one or two over the top of the foot, you always need to fit it to your foot as follows.
Open up all straps and make sure your heel to ball fits the heel to ball of the sandal correctly.
Leave forefoot and heel strap alone and use ankle strap to get in and out of shoe.
Dr. Jenny Sanders Shoe Blog 
It looks like her sandal is slighty too small for her foot.
I myself have also not tighten straps on a sandal while around the house. Wearing a pair of Chaco’s once, I almost twisted my ankle as my foot slipped sideways off the sandal.