Lead is very malliable and "flows" into all kinds of odd shapes when it hits something hard. My guess would be the "tail" was formed on impact. Lead bullets are "only" going 1,000 fps or so but they spin at 100,000 rpm (about 20X faster than a screaming car engine - a modern 5.56 NATO about 250,000 rpm! A Dremel tool only spins 18,000 rpm). When that suddenly stops wild things can happen.