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  
   reference: ["A technical atmosphere (symbol: at) is a non-SI unit of pressure equal to **one kilogram-force per square centimeter**"](https://en.wikipedia.org/wiki/Technical_atmosphere)  
(The [German Wikipedia article](https://de.wikipedia.org/wiki/Technische_Atmosph%C3%A4re) mentions the 10m water column; which is equivalent to weight of 1kg/cm²)
 - 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  
$1 Pa = 1019 \times10^{-5}Pa$

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**EDIT:**   
I'm referring above to the value of 1 **at** (called "technical atmosphere") not the standard atmosphere.

Note that the title of the original question is ambiguous as it just mentions "1 atmosphere".