According to a statement issued by Bundeswehr, the problem affecting G36 rifle was caused by bad ammunition. Specifically the thickness od copper jackets being too thin, as well as thickness of the shell casing.
Personally this seems like passing the blame. In the initial coverage of the problem, H&K stated that the rifle was made within specifications drawn up b German military, which didn't address the issue. There has been talks of some agencies in US that raised the problem. Furthermore, it is possible that the initial requirement for the rifle was intended for domestic use, not in some foreign land with exremely hot climate.
From: The Firearm Blog