Although not guaranteed, a soft fork can still result in a (lasting) chain split. Especially in the case of Bitcoin (which has slow difficulty adjustment), if the hash rate of the SegWit miners was lower than that of the old miners, the legacy chain could have easily prevailed/survived. It's a game of chance (read: race) right after the fork though, with the new nodes "winning" as soon as they generate a longer chain than the old nodes. This becomes ever more likely of course with a higher percentage of the combined hash rate.