Here is one explanation, from an article today in the Wash Post: Ancient hatreds, modern methods: How social media and political division feed attacks on sacred spaces.
Here is another from the NY Times: Synagogue Shooting Keeps Religious Leaders on Edge: ‘No One Should Be Gunned Down in Worship’
For more information about security for houses of worship, see my other website: www.disastersandfaith.wordpress.com.
Recovery Diva 
When we have a national leader who encourages violence against people he doesn’t like, and when just about anyone can buy an AR-15 or similar weapon, the violence should not come as a surprise. The hate has always been there. What’s more recent is the legitimization and encouragement of the hate and the easy availability of high-firepower weapons. I carried an M-16 in Vietnam. Nice little rifle, but it doesn’t belong on the street.