"The straw that broke the camel's back" was, according to one Gulf official, the fact that the Qataris paid a $1bn ransom for the release of 25 members of the royal family who were taken hostage while on a hunting trip in Iraq. Around $700m of this went to Iranian figures and the Shia militias they support. The rest went to Islamist movements in the region (source: Financial Times). All in all very convoluted but demonstrates that Qatar's willingness to play both sides of the Shia - Sunni street is a major source of frustration - at least to the Sunni side. 

It is also worth noting that, while all politics may be local, the LNG angle here has global implications. The Japanese, who account for c1/3 of global imports, have sought and received assurances that the flow of LNG from Qatar, the world's largest exporter, will continue unabated. In April Qatar lifted a ban on developing the world's largest gas field, a field it shares jointly with Iran.