Miss D may be right

Certain Galaxy models have a voice controlled 'assistant' called S voice.
To switch it off you need to:
Step 1: Activate S Voice on by quickly pressing the home button twice. Then, Tap the menu overflow button on the top-right followed by the Settings option.
Step 2: Scroll down to the Wake-Up category and disable the option to activate S-Voice via the home key. You can also switch off the voice wake-up functionality from here. This should help in slightly increasing the battery life as well.