Previously Poppler (app-text/poppler) used to have stable branches with even middle version number, say e.g. 0.24, and bug fix releases 0.24.1, 0.24.2, 0.24.3, ... but a (most of the times) stable ABI. This meant that such upgrades could be installed without the need to rebuild any applications using Poppler. Development of new features took place in git master or in the development releases such as, say, 0.25.1, with odd middle number; these we never packaged in Gentoo anyway.
Now, the stable branches are gone, and Poppler has moved to a flat development model, with the 0.28.1 stable release (stable as intended by upstream, not "Gentoo stable") being followed by 0.29.0 and now 0.30.0 another month later. Unsurprisingly the ABI and the soversion of libpoppler.so has changed each time, triggering in Gentoo a rebuild of all applications linking to libpoppler.so. This includes among other things LuaTeX, Inkscape, and LibreOffice (wheee).
From a Gentoo maintainer point of view, the new schedule is not so bad; the API changes are minor (if any), and packages mostly "just compile". The only thing left to do is to check for soversion increases and bump the package subslot for the automated rebuild. We're much better off than all the binary distributions, since we can just keep tracking new Poppler releases and do not need to backport e.g. critical bug fixes ourselves just so the binary package fits to all the other binary packages of the distro.
From a Gentoo user point of view... well, I guess you can turn the heating down a bit. If you are running ~arch you will probably see some more LibreOffice rebuilds in the upcoming future. If things get too bad, you can always mask a new poppler version in /etc/portage/package.mask yourself (but better check for security bugs then, glsa-check from app-portage/gentoolkit is your friend); if the number of rebuilds gets completely out of hand, we may consider adding e.g. every second Poppler version only package-masked to the portage tree.