What causes lemon meringue pie to "weep" when first cut?

Humidity will certainly affect it as littlesister suggests. Meringue contains a high ratio of sugar, which will actually pull moisture out of the air into little teardrops on your pie.

It could also be an issue of humidity in the air. The pie should cool completely before you cut it, preferably in a very dry environment. If you have time to let it cool in the oven after you turn the heat off, that could help. Meringue... such a sensitive little thing.

Or the lemon filling has too much liquid

If you're talking about the meringue weeping, the egg whites were probably over whipped - causing water to leech out. If they look a bit like cottage cheese rather than a homogenous mixture, this is the case.