This appears to be a Unity bug. Steam creates its windows with _MOTIFY_WM_HINTS set to turn off decorations. Steam itself doesn't change those hints again, but following your sequence of mins and maxes the decoration bit gets turned on, I assume by Unity itself. I'll send this on to Canonical.

Logged at Canonical.

https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1122478

Closing as Canonical believes this is a known Unity/compiz bug, #1083186 (also link above).

Canonical has reached the point of applying backported fixes to 12.04 and 12.10, so updates should be not too far away.