VODs currently default to 160p if navigated to from a Clip.
This only pertains to the "twitch.tv/channelname/clip/uniqueslug" pages, not the older "clips.twitch.tv/uniqueslug" pages.
This also doesn't happen if you click "Watch Full Video" after creating a clip, as you're on a "clips.twitch.tv/uniqueslug" page at that point.

Specific reproduction steps - follow them closely:
1. Go to any Twitch channel page
2. Click the channels avatar to expand the featured/videos section.
3. Click Videos -> Find a clip and click the clip
4. Click the "Watch Full Video" in the lower right corner just below the player
4.1 (If no "Watch Full Video" button is present on the page, there's no available VOD for this clip. Try a different clip until you find one that has a VOD available)
5. Observe how the VOD defaults to 160p

This appears to be the only case where the VOD always defaults to 160p. Navigating to a VOD in various other ways, either by manually copy/pasting the URL of a VOD, clicking a VOD directly from a channel, clicking a VOD from a recommended section on Twitch, etc. never defaults the VOD to 160p.