Cause #1: Band too loose
When the band rides up, the straps drag down. Sister-size to a smaller band (and bigger cup).
Cause #2: Straps too long
Adjust to where you can fit one finger under the strap at the shoulder. Replace bras when straps no longer adjust short enough.
Cause #3: Narrow shoulders
Look for racerback or convertible bras. Petite-cut bras also have narrower-set straps.