The main thing they look for is whether or not the Pokemon brings an imbalance to the game. This could be because it lowers certain Pokemon's usage, or limits teambuilding options. Take Aegislash for example; ever since it got kicked from OU, things like Mega Medicham and Starmie have seen a rise in usage as one of their primary counters left the tier. Other things include:
- How easily it is countered. Things like Mega Mawile really never had a solid counter, nothing could really switch it without being immediately threatened by a ridiculously overpowered attack.
- What it covers in the tier. What significant Pokemon can it stop? What Pokemon does it stop from being useful?
- How important is it to teambuilding in the format? A centralising Pokemon might be seen as an acceptable part of the metagame if it also keeps other threats in check, or has a healthy utility that could not reasonably be replaced by a different Pokemon.
Also remember that sometimes there are bans based 100% on usage. If a Pokemon reaches usage of ~3.4% of all teams in the tier above it, then it will move up at the next tier shift (which happens every three months). This is how many bans happen, Smogon only do suspects if they genuinely feel that the Pokemon breaks the game.