Informed Comment has the article Yemen Bombing: It’s not ISIL and it’s not Sunni-Shiite Conflict.
So this is political. The Houthi movement has politicized Zaidi Islam, after the Saudis politicized hard line Sunni Islam. The Houthis have all kinds of enemies now– secular Arab nationalists loyal to the Aden government, Sunnis who resent Houthi dominance of largely Sunni cities like Taizz, southern secessionists, and al-Qaeda in the Arabian Peninsula. Any of these could have hit the mosques, not because they hate Shiism but because they oppose the Houthi take-over of Yemen’s government in the north.
I don’t know enough about this situation to pass judgment on the accuracy of the above analysis. However, it is worth keeping in mind as an inoculation against what you will read and hear in the lame stream media. The lame stream media has a preconceived notion as to what this is about, so they feel no need to spend any resources digging any deeper. Besides, the lame stream media’s story helps promote more war which is very profitable to the owners of the lame stream media.