Unload due to memory pressure

Good day,

I am not a SQL DBA, but need to find out what's happening on our customer's database. Using SQL Server 2012.

I see the following messages in the error log:

2016-06-14 05:08:17.01 spid17s FlushCache: cleaned up 104806 bufs with 53316 writes in 82579 ms (avoided 20040 new dirty bufs) for db 10:0
2016-06-14 05:08:17.01 spid17s average throughput: 9.92 MB/sec, I/O saturation: 102, context switches 10920
2016-06-14 05:08:17.01 spid17s last target outstanding: 3920, avgWriteLatency 5

2016-06-14 05:30:37.34 spid19s AppDomain 7 (XXXXX.dbo[runtime].8) is marked for unload due to memory pressure.
2016-06-14 05:30:37.36 spid19s AppDomain 7 (XXXXX.dbo[runtime].8) unloaded.

2016-06-14 05:34:12.40 spid13s SQL Server has encountered 1 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [D:\SQL_Databases\tempdb.mdf] in database [tempdb] (2). The OS file handle is 0x0000000000000BE0. The offset of the latest long I/O is: 0x000001df780000

Please let me know if anyone here could tell what's wrong and where to start fixing?

Thank you,

Here is a very good article on the issue.