This does appear to be winterburn, and yes, trimming in autumn (if they started to regrow) can result in a greater susceptibility to this kind of damage. Usually, it's best to avoid any shrub trimming in autumn. July is roughly the timeframe to be wrapping-up any needed trimming because woody plants begin the process of tissue preparation for winter well before freezing weather, and since trimming usually stimulates regrowth, you want that tender growth to have plenty of time to harden-off. Boxwoods tend not to need late-season trimming anyway if pruned earlier in the year due to their relatively slow growth rate. When pruning boxwood, always try to thin the branches selectively instead of shearing them into a very manicured shape, since the resulting leaf density of the latter increases their vulnerability to pest or disease outbreaks.

You can see if any other conditions addressed on our Boxwood: Identify and Manage Common Problems page match symptoms you're seeing (since we only received one photo). For the burnt tip growth, though, all you can do is to either wait for it to fall off on its own or to selectively pinch or trim them out.

Miri