Once I read an article where they said that in our brain the centre for perceiving pain of others is close to centre of pleasure so maybe that's why. 

I remember a workshop on ¨affect theory¨ where they talked about how the excitation of the nervous system is related to emotions.  Much of what we laugh about is caused by a sudden spike of nervousness followed by a reduction in that nervousness.  We experience that reduction as pleasurable.  I assume that this is what happens: that little trip creates a brief moment of suspense and anxiety, rapidly followed by learning the person is not too seriously harmed. That quick change provokes the feeling of laughter.

If the stimuli does not feel overwhelming, the whole experience can be pleasurable.  For example, the age at which young children enjoy playing ¨peek-a-boo¨ is related the age when there start to have separation anxiety.  That tiny bit of someones face being hidden provokes just a little bit of anxiety but soon is replaced by that wonderful feeling of relief that the person is there. 

If you think about jokes, you will probably realize that most of them provoke a very, very small prick of anxiety or stimulation--- something is different than we expect, or we are making fun of death, prejudice, embarrassment, etc, folllowed by a situation that turns out to be safe and we laugh together. 

If you Google ¨affect theory humor¨ you will see some scientific articles about how humor works.  One in particular, https://www.tandfonline.com/doi/pdf/10.1080/17482620600789412 ; describes ¨how shame can power all forms of humor.¨ I just skimmed the article, but think it probably is relevant to your topic.  Watching someone trip would involve affects related to anxiety and related to shame. 

Probably because many people believe that:

'Tragedy' minus 'me' equals 'comedy'.