Very high transactions per second
Watch for the high activity again; when you see it, start a server side trace or if necessary use Profiler briefly to see what's going on.

Alternately, use a packet sniffer like Wireshark to capture raw wire activity.

Check dm_exec_cached_plans to see if that gives any idea what's going on.

Watch dm_io_virtual_file_stats to see which files in particular, if any, are being hit.