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.

No direct reason.

But some have returned out of free will, mainly to regions where Russia is not occupying. You have to consider that most refugees are women and children. Husbands and adult male children had to stay home and couldn't leave easily. So they just want to go back to their family.

I've talked to some refugees and it's heartbreaking to see a child crying and asking to go home to see Dad.