In Google Analytics Engagement Reports - Why Is Page Depth Sometimes Less Than One?
On first glance - seeing a page depth of less than 1 - the assumption is often that this must be due to bot traffic or that somehow analytics didn't load a page view properly.
In most cases however this is due to the way that analytics records sessions. After 30 minutes of inactivity a session times out (unless you amend the default tracking settings).
A session with a page depth of less than one typically occurs where you have event tracking on a site.
Lets look at an example - if a visitor reaches your site, looks at a few pages and then goes for lunch. If they return after 30 minutes (they may have left the site as is on their PC) this will count as a new session.
If this page had something on it that is being tracked via event tracking (e.g. a file download or a widget) any interaction with it will generate an event but not a pageview - if they then leave after such an interaction a session is recorded with an event but with no page view - hence a page view less than one.
Do however conduct some additional tests. Referral spam and ghost referrals are currently an issue within Analytics with some generating events based data rather than page views. We'll be writing an article on this shortly.