There are all sorts of reasons. Here's just one of them . . .
Your guy might have been working (in his capacity of director of his limited company) for someone who required a LOT of work done. As an example, let's say that it was a garden centre that wanted their extensive grounds landscaped in order to show off their products at their very best. So the landscaping business spends a fortune on hiring diggers, employing sub-contractors, etc, etc, to completely change the look of the site, knowing that the business will get its money back when the garden centre settles the bill. Then the garden centre goes bust. As a result of that, your guy's company is then also forced into bankruptcy, resulting in its dissolution.
So your guy can no longer trade as a limited company (unless, of course, he sets up a new one) but he's still perfectly capable of working as a sole trader (and quite possibly using a business name remarkably similar to his former one).
As I've said, that's just one of many possibilities but such things definitely do happen. i.e. companies go bust, and get dissolved, as part of a domino effect from one or more other companies going bust, even though they've been trading perfectly ethically and soundly up until then.