Why use 碰头 (pèngtóu) to mean "meet" instead of say 见, 见面, or 看见? (In e.g. 过两分钟我们在外面碰头) In the Chinese translation of Matilda (p. 195), we have:


  “好，你把东西收拾一下，过两分钟我们在外面碰头。”
  "Good.  Gather up your things and I'll meet you outside in a couple of minutes."


and on the next page:


  亨尼小姐在校门外和玛帝尔达碰头，两人一起默默地穿过村子的大街。
  Miss Honey joined Matilda outside the school gates and the two of them walked in silence through the village High Street.


I don't understand why 碰头 is used here, instead of say something like  见, 见面, or 看见.  I'm thinking it is perhaps more casual.

Question Why use 碰头 to mean "meet"?
Actually, the 碰 itself can mean to meet. E.g. 我们周六再碰一下吧。// let's meet again on Saturday. 

It can form the words: 碰头, 碰见, 碰面. The dictionary definition of 碰头 is 会面；短时间地聚会(short meeting). The question 'why' in any language is hard to answer and sometimes even unanswerable. For this case, we can only imagine two people run into each other and figuratively it implies they 'meet'. Usually, 碰 happens in brief time(they bump into each other and then split immediately), which reflects that the meeting is short. That could somehow explain why 碰头 can mean "short meetings". 

My two cents.