No it is **not** coincidence:  

It is because units for mass, force and pressure have been **choosen** such that common ratios of those units (density of water $\rho_{Water}$, gravitational acceleration on earth $g$; pressure unit $1 at$) have values that are powers of 10.
(Further improvements in measurement changed those values a little bit later on; especially $g$; that's why the values are not exactly $1.000\times 10^n$). 

 - 1 at is **defined** as the pressure of 10m water column
 - The density of water is **not by coincidence** (but by early definitions of unit of mass kg) almost exactly 1000kg/m³
 - The gravitational acceleration $g$ is **not by coincidence** (but by early definitions of unit of force N) approximately 10N/kg (more exactly about 9.81N/kg; even more exact value depends on the place on earth (especially latitude)).  

Putting that together you get:  

roughly:  
$1 at \approx 10 \times 1000kg/m³ \times 10N/kg = 100000 N/m² = 100000Pa$  

more exact:  
$1 at = 10 \times 1000kg/m³ \times 9.81N/kg =  98100 N/m² = 98100Pa$  
or  
$1 Pa = 1019 \times10^{-5}Pa$

_________
**EDIT:**   
I'm referring above to the value of 1 at (technical atmosphere).