So why might people post seemingly excessive concerns regarding the danger in their neighbourhood? Background: The reason for this post is that I have noticed that many people have a desire to make that which is pretty safe seem dangerous.  It is very perplexing to me.

Despite holding a master's degree my psychology education is limited to 101, yet I am still curious about a trend I am seeing.  

My neighborhood facebook page, which can be very helpful, is also scattered with posts from a subset of people who want to make the neighborhood seem dangerous.  It is a bit large, 1300, homes and really very safe.  There was some vehicle "break-ins", which was really change and loose items being taken from cars that were unlocked.  My family locked our three cars and never had a problem.  That is really the extend of the crime.

One person recently posted:  "Large box truck pulling a trailer on Xxx Pkwy. No company name. Anyone expecting home builder maintenance?".  The street in question services all the homes an an elementary school.  About 5% of residents are actually on the facebook page.  It is a public street, not gated.  How would one reasonably expect this to get any "traction".  Turns out it was a crew painting fire hydrants.  

Another example:  Guy posts a picture of a car stopped in front of his house.  "What are you doing in front of my house at 1:12 pm.  Be warned I will not be robbed."  Again this was just after lunch on a side street, but still public.  One person points out that the house across from him is for sale.  They were probably checking it out.  To my knowledge there have been no reported break ins, other then abandoned houses, in the last 8 years.

Question

So why might people post seemingly excessive concerns regarding the danger in their neighbourhood?  

It seems counterproductive as it would tend to drive down property values.
My thought is that this behavior fulfils a particular need, but perhaps I am thinking about it wrong.
@what has a great point in wanting to know the the social context. Sounds to me simply like fear, but to know the origin of the fear we need more context. A lot more.

My hypothesis is that something is giving the perception of danger to some people, either real or perceived, and they spread the mentality; some are more susceptible to it than others. 

It can also be an effort to try to keep people away, like not wanting new people moving in. In a xenophobia (also a flavor of fear) fashion:


  An excessive and irrational fear of anything foreign. This fear is most often of foreign people, places or objects. People who are xenophobic may display fear or even anger toward others who are foreign.




Knowing how these people got this perception of fear is the blurry part. In a very safe neighborhood, these claims sound slightly paranoid. People can mix some story told from family or friends, plus some media, add a bit of cultural backing, mix up with a particularly fearful personality and bang, chaos ensured. 

Given an incomplete scenario, like the tales above, our cognitive bias will fill out some details which we are missing, or we have trouble understanding, and we might have bad tendencies in what to fill. Negativity bias is a good example: 


  The negativity bias refers to the notion that, even when of equal intensity, things of a more negative nature (e.g. unpleasant thoughts, emotions, or social interactions; harmful/traumatic events) have a greater effect on one's psychological state and processes than do neutral or positive things.


Suppose a crime happened in our very peaceful neighborhood some time ago. Not everyone is used to it. It is new, dangerous and provides a feeling of unsafeness:


I'm vulnerable at home
It represents danger to my possessions
It represents danger to my family
There will be invasions to the safeness of my property.
I don't know what it will bring, but it won't be good


This is an article from UK, but features some context-agnostic points when it comes to the fear of crimes:


  Many crimes have emotional effects on
  victims that include upset, fear and anger. They can also result in psychological
  problems where the crime involves the victim coming face to face with the
  offender, particularly when violence or the threat of violence occurs
  
  ...
  
  Robbery and its effects on victims may also
  affect the feelings and behaviour of non-victims, some of whom may be able to
  appreciate its effects at first hand.
  
  ...
  
  Street robbery is predominantly an urban phenomenon, with most robberies
  taking place in streets near the victims' home and close to transport access points
  (Barker et al., 1993; Mirlees-Black et al., 1996). Though not a seasonal crime,
  more attacks occur when it is dark, with violence used in approximately 44% of
  incidents (Barker et al., 1993) and weapons involved in about a third (Mayhew
  et al., 1992). Young people represent a substantial proportion of the victims of
  street robbery (e.g. Barker et al., 1993) and, though only slightly, males are more
  at risk (Mayhew et al., 1992)


This article, also from the UK, presents studies on risk perception in the fear of crime:


  Instead perhaps
  the strongest evidence relates to
  the importance of public concerns about neighbourhood disorder,
  social cohesion and collective efficacy (Ferra
  ro, 1995; Perkins &amp; Taylor, 1996; Jackson, 2004;
  Wyant, 2008). The idea here is that the incidenc
  e and risk of crime has become coupled in the
  public mind with issues of social stability, mo
  ral consensus and the colle
  ctive informal control
  processes which underpin neighbourhood order (Ba
  nnister, 1993; Walklate, 1998; Girling, Loader,
  &amp; Sparks, 2000; Jackson, 2006).


Forgive the below-average english.