On a 757 the spoilers are used to increase drag and reduce lift in flight and on the ground. However they also supplement the ailerons in roll control depending on the amount of control wheel input. Therefore in a turn you will very like see them deploying without any direct input by the pilot other than through the normal control wheel input.
In a descent, or when slowing down in level flight, or when slowing down and descending they will be selected by manual input of the speedbrake lever. This is an almost every flight occurance at some point or other.
Finally the spoilers are fully deployed automatically ( normally ) or manually on landing to reduce lift and transfer weight to the main wheels (and brakes). This occurs as soon as the main gear trucks untilt, or when either thrust reverser is activated, or when manually applied.
It is not likely to be the case they were deployed my mistake, as to do so would be very difficult and extremely unlikely. They are used very often and normally to achieve a desired profile by either increasing a rate of descent, or increasing the drag to slow the aircraft down. The 757 is quite a slippery ship, and often needs assistance in reducing speed particularly in busy traffic environments or where absolute restrictions apply, such as in the USA (and other locations) where a mandatory 250 kts or below is a requirement below 10,000 feet for example.