[BEAR.]

At the time she was young and a new leader. Crookedstar died in A Dangerous Path - just a book before TigerClan was formed. TigerClan also wasn't named after Tigerstar; it was named after one of the three mythological Great Clans.

According to Vicky, "Leopardstar was afraid of Tigerstar - but also in love with him, poor thing". They also indicated she was prideful-- to the point where she refused to break her agreement with him even when her Clanmates were being threatened with death and her deputy was murdered. In Cats of the Clans (as shown on the Warriors Wiki), it's stated she's fearful and suspicious of half-Clan cats, and that she wasn't afraid of Tigerstar, but of her Clan losing their respect for her. It also says she wasn't aware Tigerstar was going to kill them, she was under the impression they'd be exiled, and while she would have protested Stonefur's death, she didn't because her "ambition clouded her judgment".
While she may have initially thought joining TigerClan was the best for her Clan (would make it stronger), she came to realize her mistake after Tigerstar began to control both the Clans, and regretted it for the rest of her life.