Black bar down the BACK of sheet

Hi,

can someone explain why a Canon IP5200 smears a fat (about 6 mm) black fading bar down the BACK (!!) of the sheet?

This shows up every time the printer has been pausing for > 1/2 hour or so, after 4 or 5 printed pages this mysterious ink reservoir appears to be exhausted, the sheets thereafter are clean - until after the next pause.

See photo below.

TIA

Manfred