The major factor is polling, which has seen Labour outstripped nationwide by Reform UK Ltd (which party hasn't been delivering on polling's promises in recent real elections, but the trends' effects on Labour Party morale will take a long time to settle down, if they ever do).
That polling reflects the political ineptness of the first year or so of Starmer's tenure. His government began by trumpeting about an "unexpected economic black hole" (whose very existence is debatable) left behind by the Tories and heralding an outlook that sounded too close for comfort to the Tories' adventures in austerity in the previous decade. Punitive targeting of groups such as state pensioners, not infrequently followed by belated and humiliating U-turns in the face of widespread public opposition, set the seal on a totally uninspiring, visionless approach to governing only worsened by the fact that Starmer is utterly lacking in charisma and, in public at least, human warmth and spontaneity.
During the general election Labour won, there was much talk of the "Ming vase approach", on the basis that Labour could win because the Tories were terminally discredited and unpopular, and the vital overriding consideration was to avoid saying or doing anything or making any firm commitments that might make unwelcome waves.
It shouldn't come as a surprise, but it seems to have done for surprisingly many, that the resulting government seems rudderless and flatfooted, and has very little in common with what are generally regarded as Labour traditions (apart from the well-practised one of perpetual infighting).
Faced with early divisions in the party, Starmer was very trigger-happy in retaliating against MPs who didn't follow the party whip - ironically on some of the issues on which he later U-turned.
All this, more than specifics like the Epstein/Mandelson revelations (which were just another set of examples of ineptness, albeit on a relatively grand and grisly scale), has added up to a riven party with many desperate for new leadership.
Whether a new leader will be enough to turn things around is anybody's guess, but I suppose it's unlikely to hurt more than the current turbulent doldrums.
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