It's because Wayland recently absorbed libwayland-egl. See:
https://lists.freedesktop.org/archives/wayland-devel/2018-April/037767.html

Specifically:
"libwayland-egl is now part of libwayland, and will presumably be
removed from mesa in the not too distant future."

On Mon, 14 May 2018 11:23:41 +0200
ludo@gnu.org (Ludovic Courtès) wrote:

> Hello,
> 
> Pierre Neidhardt <ambrevar@gmail.com> skribis:
> 
> > Sometimes when updating or install packages, guix encounters
> > conflicts. It's usually smart enough to fix it by itself.
> >
> > That said, conflicts seems to be a telltale sign of bad packaging
> > (e.g. duplicate files in a package and its inputs).
> >
> > Presently guix reports 3 conflicts on my install:
> >
> > warning: collision encountered:
> >   /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib/pkgconfig/wayland-egl.pc
> >   /gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0/lib/pkgconfig/wayland-egl.pc
> > warning:
> > choosing /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib/pkgconfig/wayland-egl.pc
> >
> > warning: collision encountered:
> >   /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib/libwayland-egl.la
> >   /gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0/lib/libwayland-egl.la
> > warning:
> > choosing /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib/libwayland-egl.la
> >
> > warning: collision encountered:
> >   /gnu/store/nf6py3ddvk8nsqxg7jlg0kmiqjigiqgw-gtk-icon-themes/share/icons/hicolor/icon-theme.cache
> >   /gnu/store/mg1ilfq7ajcsk12kanzsbb8jhgv7g5vm-gtk+-3.22.29/share/icons/hicolor/icon-theme.cache
> > warning:
> > choosing /gnu/store/nf6py3ddvk8nsqxg7jlg0kmiqjigiqgw-gtk-icon-themes/share/icons/hicolor/icon-theme.cache  
> 
> [...]
> 
> > So if I get it right, gtk-icon-themes is an automatically generated
> > store items for each profile and it duplicates a file found in
> > gtk+-3.22.29.
> > Is this a packaging mistake?  
> 
> No, “icon-theme.cache” collisions can be ignored.  In fact, I think we
> should not warn about them in the first place.
> 
> > Now to wayland:
> >  
> >> guix gc -R /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8
> >> | grep wayland  
> > /gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0
> >  
> >> guix gc
> >> --referrers /gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0
> >> | grep mesa  
> > /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8
> >
> > So mesa depends on wayland and wayland is properly referred to by
> > mesa. So far so good.  Same question: is this a packaging mistake?  
> 
> It could be.
> 
> Actually, the .so files are identical, but the .la files differ
> trivially:
> 
> --8<---------------cut here---------------start------------->8---
> $ cmp $(guix build mesa)/lib/libwayland-egl.so $(guix build
> wayland)/lib/libwayland-egl.so $ diff -u $(guix build
> mesa)/lib/libwayland-egl.la $(guix build
> wayland)/lib/libwayland-egl.la
> --- /gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib/libwayland-egl.la
> 1970-01-01 01:00:01.000000000 +0100
> +++ /gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0/lib/libwayland-egl.la
> 1970-01-01 01:00:01.000000000 +0100 @@ -1,5 +1,5 @@ #
> libwayland-egl.la - a libtool library file -# Generated by libtool
> (GNU libtool) 2.4.6 Debian-2.4.6-2 +# Generated by libtool (GNU
> libtool) 2.4.6 # # Please DO NOT delete this file! # It is necessary
> for linking the library. @@ -38,4 +38,4 @@
>  dlpreopen=''
>  
>  # Directory that this library needs to be installed in:
> -libdir='/gnu/store/9hddxnv7q3q819axyw0yngzppmrgmjfi-mesa-17.3.8/lib'
> +libdir='/gnu/store/2w8lhl8hyvciy3hb0h2r0mbhx7pmniy1-wayland-1.15.0/lib'
> --8<---------------cut here---------------end--------------->8---
> 
> I’m not familiar enough with these packages, and I’m not sure why they
> both provide this library.
> 
> Ideas?  Rutger maybe?
> 
> Thanks,
> Ludo’.