If the lead has moved, usually, somehow a fish is responsible, be it a fish trailing line, line 'bite', or a rig ejection.
Undertow can move a lead, and cause constant action on the bobbins though. You may need to up the indicator weight to prevent this movement. Bobbins should NOT be chosen on fashion, but on actual fishing situation, and mega light bobbins are not suitable on every water, even every swim.