what will cause subs to stand up?

I had this happen once with a bad rockford amp. the output was putting out something like 14 or 15volts or so, if not more. the subs would  hit xmax (stand up as you put it). it would be a fault with your amp, I'd guess. Can you change amps?

If you do have a loose connection it is inside your amplifier. There is nothing outside the amp that could cause this condition. If this happens again it could damage your woofer. Proper fusing of your amp power wire could possibly prevent woofer damage if it were to happen again. 

My amp wire is fused.

What kind of an amp is it? How many subs, what is the impedance of your sub, if 2 subs are they bridged?  Single or dual voice coil?  Does the amp itself have fuses on it, if so how many and what size are they?

My subs are bridged, my amp has two fuses, cant remember the sizes

take a voltmeter, put it on DC setting and see what you get when you put the positive and negative probes to the output of the amp. I'm assuming you are bridging the amp, so you would probe the bridge + and -. I dont know if this will or wont lead to a "solution" but it will give some interesting info about the situation. While you are at, you can check the ohmload of your subs to see what load you really have going to the amp.