When it's hot outside, your pores and sweat glands help regulate your body's internal temperature. Sometimes, when it's warmer outside (say, during the summer) your pores will open up to release sweat (perspiration) to cool us down (oil glands work the same way). This may be why your skin becomes more oily during the hot, summer months.
But be careful about over-washing your face. This takes away the oil your skin needs to stay hydrated (making your skin work over-time to produce more to help balance the problem). Stick to washing your face 2 times each day (morning and night) and try not to exfoliate too much!