If we take a three phase transformer with secondary connected in delta and do not connect load on one phase. Other phases will be loaded to full capacity.
if secondary side is delta connected and you put a load on only two phases, it is very clear that, two line output current will divided into three phases (coils) of secondary side winding.
So,
secondary winding current will be unbalanced. Two phase will experience half of the line current and one phase will carry 1.732 * line current.
now you have to calculate, the (1.732 * line current) is higher than the transformer phase current rating of not. If yes then transformer protection need to be operated.
This configuration is equivalent to open delta connection where you can load the transformer up to 57% only.
The primary star connected. The voltage of that phase, which is not taking current, increases. And in our case the system trips on over-voltage. Want to understand this.