1 To release tension
According to psychologists, humans can laugh to release tension and suppressed emotions. During funerals people, might be feeling tense and uncomfortable; thus they can easily burst in laughter if something unexpected happens.
2 Because of peer pressure
Some people start laughing inside funerals when they find that their friends are laughing. The mirror neurons in our brains can force us to mirror the laughs of others even when it's not appropriate to laugh.
3 Because we are alive
According to psychologists, a person can laugh at others misfortunes unintentionally because of realizing that they are OK. One of the reasons behind a laugh could be that you feel safe.
4 Because unexpected things happen
According to psychologists, any unexpected twists can make us laugh. During funerals, people are expected to behave in a certain way. Due to embarrassment and other emotions, some people might do unexpected things and thus make others laugh. (See Why unexpected things attract our attention?)
5 Because we aren't supposed to laugh
The fact that a person shouldn't laugh at a funeral might create some kind of tension. This tension can result in suppressed emotions that could then force a person to burst in laughter when the slightest thing happens. (See Why we laugh at things that are not funny?)
6 You meet close people
According to psychologists, laughter can be a form of communication between humans. During funerals, people gather together and so your chance of meeting close people becomes very high. The fact that you see close people around might make laughter easier.
7 Remembering something funny
Some people laugh at funerals when they remember something funny. This could happen as a result of a trigger that reminds them of the funny event.
8 To cope with grief
Sometimes the close relatives of the dead person laugh during a funeral. According to psychologists, laughter can sometimes be a coping mechanism that helps us deal with grief and sadness.
9 Because of Pseudobulbar Affect disorder
Pseudobulbar Affect disorder is an emotional disorder that results in uncontrolled bursts of laughter or crying that happens in an unpredictable way. People with Pseudobulbar Affect disorder might have problems expressing their emotions and so they might easily burst into laughter.
10 It's a normal reaction
Laughing can be a normal reaction to intense unwanted emotions. It doesn't mean that the person who laughed is bad or abnormal. Most people feel ashamed and embarrassed when they laugh at funerals because they truly didn't meant it.