Why would gums bleed locally a lot while using oral irrigator, but not while brushing (actually brushing stops the bleeding)?

Periodontal disease: When gums are inflamed, the slightest touch causes them to spontaneously bleed. It is the body's way of trying to get rid of the bacteria causing the disease. If you keep up brushing flossing ; irrigating, eventually the bacteria will be under control. By not doing anything, the bleeding appears to stop because the tissues are not stimulated. But you are allowing more bacteria to grow.

Gingivitis?: When was the last time you visited a dentist for cleaning? If you have bleeding from an oral irrigator, then you most likely have gingivitis unless the irrigating force is too strong. If you have not seen your dentist for more than 6 months, it is time to make an appointment now since chronic gingivitis increases the risk for cardiovascular disease and tooth loss.

Localized gingivitis: Sometimes there is a localizrd area inflammed which bleeds when flossed or irrigated in that particular area..If brush bristles dont touch that area there will be no bleeding.You should still irrigate the area even if bleeding that will help get the inflammation down as well as see your dentist to treat the underlying cause.