Class moderncv has a fixed definition of command \cventry, for example for style banking
\renewcommand*{\cventry}[7][.25em]{
\begin{tabular*}{\maincolumnwidth}{l@{\extracolsep{\fill}}r}% <============
{\bfseries #4} & {\bfseries #5}\\%
{\itshape #3\ifthenelse{\equal{#6}{}}{}{, #6}} & {\itshape #2}\\%
\end{tabular*}%
\ifx&%
\else{\\%
\begin{minipage}{\maincolumnwidth}%
\small#7%
\end{minipage}}\fi%
\par\addvspace{#1}}
with the resulting command in the cv:
\cventry[length--1]{year--year--2}{Degree--3}{Institution--4}{City--5}{\textit{Grade}--6}{Description--7} % arguments 4 to 7 can be left empty
After a look into the definition you can see that you can only place text with more than one line in the minipage, filled with argument #7 (Description). The other parts are realized as tabular, where you can not simply use \newline or \\ to get a new line ...
Therefore you should rewrite your entry to (see the numbers corresponding to #2 etc. in the definition):
\cventry{2011--2014 2}%
{Employee 3}%
{B.Sc. in Geophysics 4}%
{City, State 5}%
{6}%
{Extensive additional courses in Physics Quantum Mechanics,
Analytical Mechanics, Modern Physics 7}%
Based on my answer to a question with \item before cventry the following MWE can solve your problem:
\documentclass[11pt,letterpaper,sans]{moderncv}
\usepackage{enumitem}
\moderncvstyle{banking}
% style options: 'casual' (default), 'classic', 'oldstyle' and 'banking'
\moderncvcolor{blue}
\usepackage{showframe} % <==============================================
\newcommand*{\mycventry}[7][.25em]{
\begin{tabular*}{\linewidth}{l@{\extracolsep{\fill}}r}% <=============
{\bfseries #4} & {\bfseries #5}\\%
{\itshape #3\ifthenelse{\equal{#6}{}}{}{, #6}} & {\itshape #2}\\%
\end{tabular*}%
\ifx&%
\else{\\%
\begin{minipage}{\linewidth}% \maincolumnwidth <====================
\small#7%
\end{minipage}}\fi%
\par\addvspace{#1}}
% --------------------------------------------
% header details
% --------------------------------------------
\usepackage{import}
\name{John}{Smith}
\address{1234 Main Street}{Chicago, IL}{12345}
\phone[fixed]{+1 (123) 456-7899}
\homepage{https://github.com/john-smith}
\begin{document}
\makecvtitle
\section{Experience}
\begin{itemize}
\item%
\cventry{2011--2014 2}%
{Employee 3}%
{B.Sc. in Geophysics 4}%
{City, State 5}%
{6}%
{Extensive additional courses in Physics Quantum Mechanics,
Analytical Mechanics, Modern Physics 7}%
\item%
\mycventry{2011--2014 2}%
{Employee 3}%
{B.Sc. in Geophysics 4}%
{City, State 5}%
{6}%
{Extensive additional courses in Physics Quantum Mechanics,
Analytical Mechanics, Modern Physics 7}%
\end{itemize}
\end{document}
and the resulting pdf:
