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 often it is used in its tier and the tier above it. This can provide an idea of how it will fare in its new tier if it gets one.
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.