I've also had the issue of booking a wholly run TK ticket on UA stock through UA.com only to find out 12 hours later they cancelled the whole darn thing. Still to this day I have no idea why.

TK is a partner that requires 24 hours to confirm flights (unlike LH, LX, CA and some others that can ticket immediately). So when waiting the 24 hours for the flights to confirm you can easily get a cancellation notice. It could be a "data transmission error" between UA and TK, it could be TK just flat out denied the ticket sale or it could be a whole host of other reasons.

Bottom line...when booking non-UA flights on UA.com be sure you have ample time in your schedule to watch and make sure the flights ticket as booked and have a backup in case the flights do not ticket.

-RM