She is talking about the past event, but why "will" is used here? Is it different from just saying "sometimes the light come on" without "will"?
Since she says "sometimes" we know that she is referring to an event which has happened in the past and continues to happen in the future, so she must use past progressive tense in this case. The word "will" can be used for progressive tense because it can describe the result of an action or an event which is expected to take place again in the future, and has already taken place in the past. You can just say "the light comes on" instead, but "will" is also correct in this context. Ex: Sometimes when I eat before exercising, I will feel sick.