What causes bra straps slide down?

Sounds like it's too big. If your bra can move around freely it can't support you. You will need to get fitted and get a new one as soon as possible. Remember, if you put on a bra at the shop, you SHOULDN'T be able to put it on the tightest hook. What you do it put it on the loosest hooks so that after you've worn it awhile and it stretches, you can where it on the middle clasp. Adjusting the straps are important too, but more of the support your breasts get is from the band, so be fussy and make sure you know your size. Your bra SHOULD be tight, even leaving a bit of a mark is alright, of course as long as it doesn't hurt. If your bra leaves no mark, it is too loose. ( Just like the elastication on knickers leaves a mark sometimes, but fits.) Good luck with getting a new bra. Mogeta.