Which direction should QR lever point? A manual of a suspension fork states that the quick release handle must be in horizontal position, pointing backwards.

Is there any technical reason for this? Another natural position can be pointing upwards along the fork blade, and I cannot think of a reason why it is inferior.
Pointing it forward or downwards would be problematic as the lever might get caught somewhere and therefore get opened.

Remains pointing it backwards and upwards parallel to the fork. When pointing it parallel to the fork blade, it might touch the fork and therefore not fully close. This might cause the quick release to open by itself unintendedly. One can easily mitigate this if one knows about it and orients the lever a bit different, however, one has to be aware of it. Pointing the lever backwards does not suffer from this problem and therefore is the most safe version (also from a legal point of view) for the manufacturer, as one does not have to be aware of any implications.