Speculation, but I think higher bit rate = better quality/detail. Youtube's encoder is limited in what it can do. If you give a high quality/detail video for youtube to encode, it cannot keep up and it just craps out.
I tried over 200 different rendering permutations on both sony vegas and adobe premiere. MP4 containers, h264 encoder, wmv container, VBR, VBR2, CBR, 8mbps, 50 mbps, raw, different work paths (like using handbraek or axidemux), and various other settings.
There are some ways to mitigate it, but its not really worth it.
What container/encoder are you recording to and what recorder are you using?