Power Supply Orientation Affecting Output
Industrial DIN-mount power supplies are often passively cooled by convection.  That is, they are designed so that the hot air rises out of the enclosure, drawing cooler air from underneath.  The placement of ventilation holes, internal component orientation, internal baffles, etc, all play into the design.
If you choose a different orientation then it doesn't work so well!
Also, these power supplies will specify minimum clearances around them.  These clearances must be met for the same reason.
This particular power supply's data sheet hides the clearance requirements on page 2-7, in an unfortunately-named &quot;Cooling&quot; section:

Cooling
The NI PS-14 is convection cooled, and direct cooling is not required. However, you must not cover the ventilation grid (for example, with cable conduits) by more than 30%.
Proper installation clearance for the NI PS-14 is 40 mm on top, 20 mm on the bottom, 5 mm on the left and right side when loaded permanently with full power. If the adjacent device is a heat source, 15 mm clearance is recommended between the NI PS-14 and the adjacent device.