Sudden increase in Page Writes/s after patching / rebooting
When you restart the service or reboot, the buffer pool and plan cache get wiped out as a result, and so when the server starts back up again, they don't get loaded until users start executing queries. 

What you can do to mitigate this is have a stored procedure aimed at simply running some queries that will load up your most frequently-used data into memory. Call that procedure after a restart, and you will feel that initial delay instead of your users.