Can an employee abuse the seniority system and bullies the other Employee bec of these senority systems?

I am so confused about this seniority system. I was working night and just come aboard for this company last year and my manager transferred me in the Morning and this is it started. We all making the same amount and it doesn’t matter if you are new or been there as Employee 20 years ago. The point is , these Morning shift , they are abusing it. Like, if I started to work at 8:00 am, to stock the freezer foods, and these people come at their regular schedule at 9:00, they are boiling pisst Off bec. I was coming at 8:00. They are saying , they should come at 8:00, and stock the freezer . So, I was step down to my used to be hours 9:00, and not doing the frozen which I started doing this and they never did. They’ve been there 8 , and 16 years but they stocked the frozen.
Make story short, they start coming at 8:00 , but they never do their job to stock the freezer. So, is this really a senority issue? Thank You
Ms. Confused