Triggered extensions just mean that the player automatically has an 'offer' in front of them - the offer of the current contract being extended. They don't have to accept that offer though and are likely to discuss their position at the club before doing so.I think it was announced that Westhoff had triggered the one year extension, not signed the one year contract. You seem to be able to get out of triggered extensions if it suits the player (and club).
Another example described in the recent AFL.com.au article is Casboult. "Casboult triggered a one-year extension in his contract after playing 30 games across the past two years, however he is still expected to look for more security before agreeing to exercise that clause."
So some players may choose to look elsewhere before agreeing to the terms of the contract extension they've triggered. Westhoff is probably considering if he actually wants to play on or not, not looking elsewhere.