When and who shrunk the ldf file?
Below is a sample query to return information from the default trace using T-SQL.

SELECT  
     trace.DatabaseName
    ,trace.ObjectName
    ,te.name AS EventName
    ,tsv.subclass_name
    ,trace.EventClass
    ,trace.EventSubClass
    ,trace.StartTime
    ,trace.EndTime
    ,trace.NTDomainName
    ,trace.NTUserName
    ,trace.HostName
    ,trace.ApplicationName
    ,trace.Spid
FROM (SELECT REVERSE(STUFF(REVERSE(path), 1, CHARINDEX(N'\', REVERSE(path)), '')) + N'\Log.trc' AS path
    FROM sys.traces WHERE is_default = 1) AS default_trace_path
CROSS APPLY fn_trace_gettable(default_trace_path.path, DEFAULT) AS trace
LEFT JOIN sys.trace_events AS te ON 
    trace.EventClass=te.trace_event_id
LEFT JOIN sys.trace_subclass_values AS tsv ON
    tsv.trace_event_id = EventClass
    AND tsv.subclass_value = trace.EventSubClass
ORDER BY trace.StartTime;


By filtering on WHERE TEXTDATA LIKE 'DBCC SHRINK%' you will find all Shrink events.