Pixma 890 randomly uses rear tray

[kenkarno]

Occasionally my Canon Pixma 890 tries (and fails) to print from the rear tray, even though the preset is set to Main Tray. If I fiddle with the presets eventually I can get it to work again, but it can take a lot of time.

I'm running on an iMac / Mavericks, although this happened with previous versions of MacOS as well.

I just printed two documents in succession using the same presets:
Word document: printed successfully from main tray
Excel document: tried to print from rear tray and failed

The rear tray door is closed.

Any ideas? This is driving me crazy!!

Thanks,
Ken