It seems to be one of those immutable laws of brand marketing that the smaller the brand the greater the propensity to cycle through new advertising agencies. It's as if the brand is seeking the magic bullet that will cure-all ills often in an effort to overcome a brand or product proposition which is just not viable. Sadly it never works out. The pattern is well established;
3 months search and pitching, hiring and then 3 months of bedding-in of the new agency, 9 months of agency / client true love, 6 months of emerging strains, 3 months of client skulking behind the scenes being wooed by other agencies, termination and 3 months search and pitching.....(repeat)
Brands that cycle through new strategic agencies every 2 years need to seriously question if there is not something more fundamentally wrong with their approach and even the need to have a strategic / lead agency at all.
Of course often the break in the relationship comes about because a new senior marketer is hired and they feel the need to mark their arrival with a very visible action. Both a regular reality and a cliché. It's always tremendously disruptive to the activation cycle. The marketing department turns inwards to refresh everything at the very moment when the stakeholders are clambering for support. Momentuam is lost and brands NEED momentuam.
If you have a have a bad agency then fire them, without hesitation (NB also ask yourself why they were hired in the first place) but if you cannot put your finger on why you are cutting loose then question your motives. It's costly and rarely results in the bump you expect AND the brand needs.