Why does dimethyl ether have a bond angle bigger than 109.5 degrees?
In case of $\ce{Me2O}$, the positive deviation of the bond angle can be explained on the basis of steric crowding of two methyl groups. 

There are other contradiction too like this:

In case of $\ce{(SiH3)2O}$, the huge bond angle increase is mainly due to back bonding and partly due to steric repulsion.