Maybe use this default trace query to get a feel for when log file growth events are occurring, the frequency, and what process is triggering it.

select
  te.name as event_name,
  tr.DatabaseName,
  tr.FileName,
  (tr.IntegerData * 8) / 1024 AS GrowthMB,
  tr.LoginName,
    tr.SPID,
  tr.StartTime,
  tr.EndTime
from sys.fn_trace_gettable(convert(nvarchar(255),
 (select value from sys.fn_trace_getinfo(0) where property=2 AND value IS NOT null)), 0) tr
inner join sys.trace_events te on tr.EventClass = te.trace_event_id
where
tr.EventClass in
(
92, -- Data File Auto Grow
93, -- Log File Auto Grow
94, -- Data File Auto Shrink
95 -- Log File Auto Shrink
)
order by
EndTime desc;