Purpz falling over?

Two of my plants are have taken to leaning over and I can't quite pinpoint the underlying problem. As you can see I already had to tape one up as its stem snapped when it fell over(has been 3 days since it was taped up). The other plant on the far right is also leaning but only started a day ago and is fluctuating up and down over the last 30 hrs or so.

Would appreciate any help you guys might be able to offer.

using advanced nutrients micro grow bloom combo, feeding 3 times a day for 30 mins. 0.5gph drippers. nutes are at 1/2 str

ive had plants do this to me before as well, the only conclusion that i could come up with was just because of the streching, so i turned off my fan and lowered my lights and about 3 days later they fixed themselves.

i did move the fan so it didnt blow directly onto the plants and that seems to have helped slightly