A leaning basket often starts because of one minor inconsistency that keeps happening. One of the spokes is slightly further apart from the others than it should be. You are pulling a little harder on the over-under weave of one side. One rod bends easily and then dries quickly into a shape as one turn follows the other. As the side wall reaches a certain height, the lean of the basket is obvious and can be thought of as a big mistake, but it is likely the result of a tiny difference in spacing or pull.
Check the basket’s shape early on, while the base is flat on the table. As the weaving progresses, the spokes should be roughly the same distance apart and the basket base should look like there are no uneven pulls on it. If you see one section has the spokes close together and another section the spokes are further apart, chances are this imbalance will remain as you pull up the side walls.
The other place to check the basket is as the sides start coming up. Often in these early stages of learning how to weave baskets a beginner will pull the over-under a bit hard on the side closest to their stronger hand. The rows might look good close up but the side will slowly start to tilt. While the weaving, take a moment to step back and look at your basket from the side. The side wall does not need to be a perfect straight line, just all of the side stakes should look roughly on a line.
One practice exercise would be to weave three or four rounds of weaver, then step away from the basket. Put the basket down, turn the basket a quarter turn, and check it for shape from different sides. If there is one area that seems to be a bit loose, while the basket is still wet, gently push it back into shape and add more rows of weaving. The more often this checking in of the basket is done while the weaving is still going, the less likely it is for the basket to get out of shape and hard to correct once the basket wall is solid.
For most beginning baskets, the main culprit to the lean is tension. Looser tension makes the basket walls loose and wobbly and very tight tension will cause the basket wall to pull and look uneven. The goal is to have the over-under rod rest next to the row below rather than pulling tight to the last one. As the over-under goes around one stake, bring it close enough to the last row that it sits next to that row, but let it still have its normal amount of spring. Even though it willow is flexible, it does still want to go back to its own way, so keep that tension in mind when working.
Sometimes a basket will get a bit out of shape while it is drying. If you are having issues trying to bend the one rod that has started to dry while you are still weaving a row, you may be putting a little bit too much force on it to keep going which can then cause the lean. Keep a spray bottle handy so that you have moisture readily available in case the rod feels hard, stiff, or very sharp in its bend. Sometimes you have to just stop to let the basket dry some more to keep that lean from getting worse.
As you weave, check your basket from the top and side. Check it for balanced tension, if the stakes are upright, and if the bottom of the basket is evenly centered as the side walls come up. If a basket doesn’t need to be perfect for you to gain a sense of it when you work it, try to catch any lean you are seeing a little earlier than you would have previously. Your fingers are picking up on it.