Most likely, as suggested, a worktable is being used, either because the dataset exceeds the available space in RAM, or because it has to do something with it that is best done in tempdb, like sorts, hash joins, etc. Would have to see more details of the execution plan to suggest something more specific, but writes to worktables during Select queries aren't uncommon, nor are the usually something to worry about.

The only potential concern I see here is that, if it's due to memory pressure forcing data into tempdb because it can't fit in available RAM, and that happens with any frequency, then it might be worthwhile to invest in a memory upgrade for the server involved.

If it's a query that will only be run infrequently (or even just once), and the server is otherwise doing fine, then it's probably a non-issue and you can ignore the writes.