Some examples of the reasons for real changes on readers-per-copy are shown below
Examples of sudden changes:
Heavy promotional activity (e.g free gift)
Price cuts
Major changes to editorial format
Sudden losses or gains in circulation
Seasonal factors
Examples of gradual changes:
Awareness of new titles can take time to grow
Long-term expansion or contraction of circulation
Changes in profile (e.g. ageing readership)
Launch and closure of competitive titles
Impact of digital media
There are many other possible reasons why readers-per-copy can change over time. A summary as to why publications have different readers-per-copy can be found here.