It sounds to me like you are stalling one wing first. If one wing stalls while the other is producing lift, it puts you into a snaproll. This is likely to happen when coordinating inputs (combining elevator, aeliron, or rudder input), but too much elevator input while even slightly banked, can also cause this to happen. Keep in mind that when you increase bank angle, you also increase your potential stall speed. One thing you can do to lower your stall speed is to deploy flaps, and retract them as your speed increases.
While this is most common at speeds near stall, it is also possible to induce a similar stall at higher speeds with an accelerated stall.