What caused the database [.mdf file] to grow suddenly
This will find all of the autogrow activities in all of the errorlog files still in active sequence, from my answer here:

DECLARE @path NVARCHAR(260);

SELECT 
   @path = REVERSE(SUBSTRING(REVERSE([path]), 
   CHARINDEX(CHAR(92), REVERSE([path])), 260)) + N'log.trc'
FROM    sys.traces
WHERE   is_default = 1;

SELECT 
   DatabaseName,
   HostName,
   ApplicationName,
   [FileName],
   SPID,
   Duration,
   StartTime,
   EndTime,
   FileType = CASE EventClass WHEN 92 THEN 'Data' ELSE 'Log' END
FROM sys.fn_trace_gettable(@path, DEFAULT)
WHERE EventClass IN (92,93)
-- AND DatabaseName = N'AdventureWorks'
ORDER BY StartTime DESC;


Now, once you've identified a suspect autogrowth event, you can see information like the application name and host name that caused the event. It's possible that you might capture other activity by the same SPID, but you can't rely on this. Just take a look for things that started or ended within an arbitrary window - this looks at 5 minutes before and 5 minutes after, and hard-codes the SPID observed above:

SELECT * FROM sys.fn_trace_gettable(@path, DEFAULT)
  WHERE StartTime &gt;= DATEADD(MINUTE, -5, '2018-03-19 11:41:16.970')
    AND EndTime   &lt;  DATEADD(MINUTE,  5, '2018-03-19 11:41:16.970')
    -- AND TextData IS NOT NULL
    AND SPID = 63;


If you are rolling through 20 errorlog files per day, something is not configured correctly or you are performing way too much of something that is filling those log files with noise. IMHO.