Tempdb growing, however it has a lot of free space

On a sql server dedicated to a Dynamics CRM database, I've been seeing some odd behavior.

While monitoring growth using the following query as basis, I've noticed tempdb has grown quite frequently in the last few hours:

DECLARE @path NVARCHAR(1000)
SELECT @path = Substring(PATH, 1, Len(PATH) - Charindex('\', Reverse(PATH))) +
                      '\log.trc'
FROM   sys.traces
WHERE  id = 1
SELECT databasename,
       e.name   AS eventname,
       cat.name AS [CategoryName],
       starttime,
       e.category_id,
       loginname,
       loginsid,
       spid,
       hostname,
       applicationname,
       servername,
       textdata,
       objectname,
       eventclass,
       eventsubclass
FROM   ::fn_trace_gettable(@path, 0)
       INNER JOIN sys.trace_events e
         ON eventclass = trace_event_id
       INNER JOIN sys.trace_categories AS cat
         ON e.category_id = cat.category_id
WHERE  e.name IN( 'Data File Auto Grow', 'Log File Auto Grow' )
ORDER  BY starttime DESC 

Gives several lines of growth of the data files:

tempdb  Data File Auto Grow Database    2015-03-02 09:50:33.187 2

However, when I look at the space occupied by tempdb I'm not seeing where the space crunch is:

USE tempdb 
GO 
SELECT DB_NAME() AS DbName, 
name AS FileName, 
size/128.0 AS CurrentSizeMB,  
size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS INT)/128.0 AS FreeSpaceMB 
FROM sys.database_files; 

Gives the following output:

DbName  FileName    CurrentSizeMB   FreeSpaceMB
tempdb  tempdev     7500.000000     7492.625000
tempdb  templog     156.132812      93.820312
tempdb  tempdev2    7250.000000     7245.625000
tempdb  tempdev3    7250.000000     7245.312500
tempdb  tempdev4    7366.500000     7360.875000

The person maintaining the CRM has asked for us to shrink the tempdb.

My colleague has, in the past, obliged. However I'm unwilling to do so without an explanation. Especially considering that the shrinking has become an almost weekly occurence.

Can anyone give me an indication as to why the tempdb has so many growth events, and how to handle this properly?

I'm currently considering asking for more storage, and increasing the tempdb drive by 50%.

However this feels like treating the symptom, not the cause.