Any idea what causes them [seiches]? That's the part that escapes me.

You got me interested, faldage, and you'll find in the links below what is summarized here.

Waves (however they are caused initially) may happen to occur with a time-period, between them, that matches the natural resonant frequency of the lake or harbor. In that case the successive waves (including the waves "rebounding" from the shore) will reinforce each other, building higher; the result is called a seiche. Its defining characteristic is thus not what has caused the initial waves, but the fact that they have "piled up".

Every enclosed body of water has a number of natural resonances. If you sit in a bathtub part full of water and rock back and forth you'll find that at the right period (about a second) you can easily get the waves to grow until they overflow the bath. The resonant oscillation of the water is a seiche.

Swiss seismologist F.A. Forel brought the word (pronounced "sigh-shh") into science in 1890. However, it seems the word was commonly used in German-speaking Switzerland to mean oscillations in alpine lakes.

A tsunami is a wave caused by underwater seismic activity, such as an earthquake or and erupting volcano. A tsunami can be massive and, if it hits the shore, incredibly destructive.

http://www.soest.hawaii.edu/GG/ASK/seiche.html
http://oceanworld.tamu.edu/text/students/waves/waves1.htm, and continue to successive pages.
http://www.weathernotebook.org/transcripts/2001/04/18.html