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That's one of the many reasons I dislike Newtonians. It does happen with refractors, but less often.
You have light getting in from somewhere. Could be indirectly from the bright moon, reflecting off the front of your tube / shield. It's more obvious now that you are pushing the envelope. The deeper you go, the more you will notice things like this happening. Something that was minor before, now becomes much more noticeable. Even the slightest glimmer of light from something (I think probably moonlight in this case) will stretch out bright along with the image.
It's a result of you progressing Ray as you advance in astrophotography. If you want to progress further, your going to have to learn new Photoshop tricks. Flats probably won't help this sort of thing, but are a good idea anyway to minimise uneven field illumination. DSS is for basic processing and can't handle uneven fields. Photoshop can, even old versions. Software flat fields in photoshop are even more essential if you don't shoot real flats. However heavily processed images from DSS are no good to photoshop. To avoid the bright areas, you have to move away from processing in DSS and process in third party software. Either astronomy friendly software or learn photoshop skills.
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