Channeling disrupted while still protected by shield?

Hi, I’m leveling a priest right now.

Usually when I have power word: shield up, when I am am channeling mind flay and get auto attacked by mobs the channeling isn’t disturbed, but sometimes even though the shield is still up my channel time gets reduced when hit by the auto.

Anyone know why sometimes channeling is protected by the shield and sometimes it is not?

Even if one damage gets through the shield, you will suffer the full pushback.