There's a chance not all 51% attacks are reorg ones though as some may try to change other things
Those other things also require reorgs. No reorgs means always extending the longest (most worked) chain, which is the honest behaviour and thus cannot be called an attack, even if one were to have > 50% of hashpower.
With 51%, an attacker can do these things:
1. Reorg previous blocks, removing (and optionally replacing) transactions that have already been confirmed.
2. Censor any or all transactions.
3. Claim all of the block rewards.
2 requires re-orging other blocks that include the txs one wants to censor.
3 requires re-orging ALL other blocks.