CT can tax non-residents on CT-source income.
CT law is that CT-source income includes income "attributable to compensation for services performed in Connecticut or income from a business, trade, profession, or occupation carried on in Connecticut." See page 10 of this reference:
https://portal.ct.gov/-/media/DRS/Forms/2019/Income/Booklets/CT-1040NRPY-Online-Booklet_1219.pdf
Since your severance pay is attributable to work you performed in CT, it is taxable by CT.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
