I thought I was going to be able to answer this question for you in a swift and efficient fashion, but it turns out to be bewilderingly complicated. It's a very odd situation, but it's a consequence of titles already held by members of his extended family. I think I've figured it out.

The convention for a viscountcy is to create it in the name of the holder's surname (Viscount Smith), the family seat (Viscount London), or both (Viscount Smith of London).

This would have been hard in Alan Brooke's case for a number of reasons.

First, it's important to remember that he was granted a baronetcy in 1945 before being created a viscount in 1946. The trouble is that there was already an existing baronetcy for the Brookes of Brookeborough; Alan was himself the sixth son of the third baronet (Victor Alexander Brooke, d. 1891), and already far out of line for any hereditary right to the title. The title was then being held by his nephew, Basil Stanlake Brooke, the Prime Minister of Northern Ireland. As there was already a Baronet Brooke of Brookeborough, then, the easiest convention available in Alan Brooke's case would likely be to combine his two names together (Baronet Alan Brooke of Brookeborough). Literally combining them into one word would be a matter of convenience to distinguish him from the existing Baronet Brooke of Brookeborough.

While there was as yet no Viscount Brooke or Viscount Brookeborough in 1946, this was a possibility that could not unfairly be withdrawn from the existing line of the Brooke of Brookeborough baronetcy -- and, indeed, Basil Brooke was created Viscount Brookeborough in 1952, with the viscountcy continuing down to today. With the Alanbrooke baronetcy already established, creating Alan a viscount in that name was the easiest solution to the problem.

I'm sure there would be an easier way for me to express this, but I hope this adequately conveys the gist of the problem. Please let me know if it doesn't!