This is a classic problem. The axle cannot be protuding outside of the dropout. Not at all. It should be about 1.5-2mm short of flush with the outside surface of the dropout. If it's flush, or protuding, the axle will pull out when you lean down on the crank on that side. There are a couple of things that can cause this problem. One, your axle may not be centered between the locknuts. There should be exactly the same on both sides and (once again) that amount must be less than the thickness of the dropout. If the two sides are not equal, then you will need to readjust everything on the axle to eliminate the offset. Two, it may be that for some reason the axle is too long. Did you replace it? If it is too long, then cut it down so that does not come flush or protude. A third possible problem is that a spacer or washer did not make it back on to the axle during reassembly. Measure the distance between the outside surfaces of the locknuts. On most bikes it should be 130mm. Some MTBs will be 135mm. Older road bikes maybe 126mm. You can measure between the insides of the dropout to find out what it should be (if that, for example, measures 131mm, then your hub should measure damn close to 130mm). There are likely more possible reasons, but I believe these would be the common pitfalls. If the "flush or outside" rule is violated, no amount of tightening of the QR is going to fix the problem. You'll likely end up stripping the adjustable side.