I'm a charity shop volunteer - I see a LOT of donations. Obviously we don't hear the stories behind every donation but there are a few main groups.
Sale bargains - we see a lot of stuff still with tags on, usually bought in the sale. Priced an M&S per una dress the other day, the original selling price was £45, it had been down to £4.99 in the sale, someone had obviously got it home and decided it didn't fit or they didn't like it that much after all. We're very suburban and they obviously felt that the trek back into town wasn't worth it for a fiver. So in the charity shop bag.
Wear it once and move on - we see a lot of party clothes, prom dresses, wedding outfits. (Although not so much this year obviously). People don't want to wear the same outfit for repeated weddings, and only go to prom once, so buy an outfit then sent it to the charity shop. This does mean on the flip side we always have lots of these outfits, worn once, as a massive bargain for other people.
Seasonal clear outs - lots of people appear to have big clothing clearouts twice a year, end of summer and end of winter, ditch what they don't want to keep for the next year. Tends to be "younger" brands like topshop, zara, h&m where the wearer knows fashion will have moved on and doesn't want to be wearing "last year's stuff".
People going into care / dying - we often get big bags of stuff from people who are moving parents into residential care or clearing houses after a death. Lots of it goes for recycling but there are often some very good vintage pieces which sell very well, jewellery made with precious metals/stones or good quality pure wool coats.