To add to my earlier post, sliding forward on the saddle AND having weight on hands suggests too much reach, too high a saddle, or both.

Too much reach means you're too stretched and are pulling yourself forward (not consciously) in order to compensate. Having too much reach can also lead to effectively having too much saddle height, because the excessive forward lean is causing the posterior muscle chain to stretch, which decreases mobility for pedaling, which makes the saddle too high.

On the other hand too much saddle height makes you reach down for the pedal which can cause two types of sliding issues. First is you move forward on the saddle to compensate. Second is that instead of supporting yourself on the pedals through the whole pedal stroke you end up "hanging" by the pedals at the bottom which'll pull you forward on the saddle until you're supported again.
If your saddle height is correct you should be pushing yourself back when pedaling.

Too much saddle height can also lead you to tilt your pelvis backwards in order to alleviate the excessive stretch at the posterior chain muscles. When you tilt your pelvis back, your effective back length shortens. If you're dancing at the edge of too much reach already, tilting the pelvis back will not help.

Saddle height directly correlates to the feeling of having too much reach or having weight on hands. If you're not supported by the legs, you'll be supported by the pelvis. That means that instead of having a support position in front of your center of gravity (ie. legs), you'll have one behind it (pelvis). When that happens there's just no way to support your upper body properly which leads to your weight resting on your hands and butt with the legs spinning on air.

Ok that's an extreme example, but it does outline the issue. It can be major or slight, but in every case it should be corrected.