If they were being honest, the actual reason is that circular avatars have been trendy and the norm for new designs on the web, iOS, and Android for the past 3 to 4 years, and the two primary reasons they became popular were because border-radius became supported widely enough around that time, and because photographs of faces arguably look better in a circular frame, at least according to current tastes.

Here's a quora thread from back in 2013 discussing it:

http://www.quora.com/Why-are-many-UI-designers-using-round-p...