Do tigers normally attack prey or people from behind?

The answer is yes, when possible. I have two sources for that. The first one is probably the best. It comes from my book Wild Cats of the World by Mel and Fiona Sunquist, perhaps the best book on wild cat species you can buy. In their section on "conflict with humans" they state that:

"The most significant reduction in human deaths followed the 1987 introduction of inexpensive rubberised masks, which are worn on the back of the head. The authorities reasoned that as tigers normally attack prey from behind, rather than face to face, a mask worn on the back of the head should be a deterrent".

Human wearing a face mask on the back of the head for protection against a tiger attack. The photo is by Raghu Rai of Magnum.

They conducted an experiment to prove that this was correct. About 2,500 masks were issued to approximately 8,000 workers entering a tiger reserve. The authorities said that "The results were dramatic".

In 1987 nobody wearing the masks were killed by a tiger but 29 people not wearing the masks were killed.

Although, some of those killed had been wearing a mask. They discovered that these people were attacked by tigers when they temporarily removed the mask! The particular source of that information is Metapopulation structure of tigers in Thailand. And Riding the Tiger: Tiger Conservation in Human-Dominated Landscapes. The authors of various. Mel and Fiona Sunquist refer to this work.

The other reference source which supports the view that tigers usually attack prey from behind is the Nature Safari India website. They state that:

"Tigers usually attack their prey from behind, especially when they [humans] happen to be sitting."

What they are saying is that a tiger might attack a person if they are sitting with their back turned to the tiger. Under these circumstances the tiger might mistake a person for a prey animal and attack. 

Enraged tiger leaps up to the top of an elephant carrying tourists. Image: Screenshot.

They advise tourists to stand up. This tells the tiger that the tourist is human and not a natural prey animal which helps to protect them. 

If in addition, if they wore one of the masks which you see in the photograph on this page, their protection would be almost complete.