There was no decree or anything, but most of Western Ukraine has been fairly safe for the past few months (relatively speaking). If you lived in say, Kyiv, and fled the country when the war broke out, you might be thinking "well I'd rather be living in my own house, in my own country, than burdening the people of (country) and living in a hotel".

They're being asked to not do so, since the extra burden to the power grid would probably cause widespread blackouts which would be shit for everyone.