Temperature and photo period (number of light hours in a day) are similar enough between spring and late summer that plants will react similarly by starting to flower in the late summer as if it were spring. Even now, some of my tomatoes and peppers are starting to flower again.
It is not abnormal. But it can vary between years if it happens at all, a little or a lot.
In fact, I often start many vegetables in August, as they will grow through the light and temperatures periods of Autumn as if they were entering summer. The yields are lower than a summer crop, but I still get quite a lot of food from this late summer sowing.
In any case, you should cut them off so the plant can concentrate on ripening its existing fruit. Be they grapes, tomatoes or peppers.